I'm not a physical therapist, so anything is pure conjecture at this point. You may have hurt a muscle or the connective tissue, it could even be a stress fracture type injury where the bone has a microscopic break or crack. Rest could be an answer, but I'd also consider soft tissue work.
This would be working the whole anterior, medial & posterior shoulder/scap & chest area, not necessarily on the pain point at all. Ideally a sports massage therapist, but you can do soft tissue work yourself with a foam roller. I'd also work at stretching the area from a variety of angles. Overly tight pec or shoulders can cause issues, so make sure the whole area is lose. I would think about posture & exercise choices. Are you working the T-shirt muscles more than the posterior chain? If so it might be time to focus on those legs & back for a while & just maintain the chest & arms.
Posture issues are the usual if you look at the rounded back posture,you'll see it sets the pecs & shoulders up for tightening right up, while at the same time over-extending the back muscles - so think posture as often as possible.
Most common is probably soft tissue, so work on that hard (avoid the painful area, work everywhere else once pain has gone simply include it). If you've focussed on t-shirt lifts, focus on balancing up by a cycle for the back & legs while just maintaining the chest & arms. Also think about nutrition you need building material to repair as quickly as possible so think about being especially sorted with your diet. Drink extra water, eat a lot of whole foods, eat a lot of green leafy veggies. It's probable that whatever your injury is there is some inflammation, so having some berries & other anti-inflammatory foods might be a good move. You can also consider icing the area after training it as well (it can be uncomfortable there as it's near your heart & other organs, but it may help, just don't ice too aggressively!). If you can see a specialist then do that, if not, just do the best you can, stop doing exercises that aggravate it - do variations that cause no pain - working through pain is stupid & can lead to chronic injury, pain is the body saying STOP, so listen to it. This doesn't mean you have to stop forever (you may have to but most times it doesn't mean that), just back off, stretch, sort tissue work, allow to heal, then in time you can go back to those exercises, start light & slowly work them back in. If you take a break from the painful moves the chances are good that you'll be able to go back to them, if not, then the chances are good you'll have to give them up for good at some time in the near future.
Unfortunately, in the past I've tried to tough it out & because of that I have issues that are now semi-permanent in nature, don't follow my path, take the time, spend the time repairing & you'll be 100% in no time.
Here's a sample day
AM
get-up soft tissue work using foam roller & tennis ball
Stretching (focus on chest, shoulder stretches)
pre-workout
soft tissue using foam roller tennis ball
mobility/activation & dynamic warm-up
Workout
Post workout
Stretch (spend most time on the chest, shoulder stretching)
Evening
Soft tissue & stretching (again)
Once every week or two massage.
You can use low power ultrasound to aid healing as well if that's available (I wouldn't use higher powered units on the chest, but if you have a home one that should be ok)
Supplements you might want to take are:
vitamin c (aids repair)
cissus (if you're not a drug-tested athlete. If you are this gives a false positive, so steer clear)
DHA/EPA (from algae, not fish) cell membranes are made of it, so having it easily available could help recovery
vitamin D (if it's a bone issue this will aid bone repair, vit D also improves immune system function & even interacts with your genes)
vitamin K2 (I always suggest vit K2 with D as it directs bone deposition to the correct site - so you don't get calcium build up in the blood vessels for example - which has been seen when vit K2 is lacking)
B12 you should always have that
Multi-vit/min to cover anything you've missed.
ALl that should get you back on track in no time.