You have been starving your body if you been eating that few calories over that time period. You probably burned off muscle and then when you added some calories it still wasn't enough to gain back your muscle loss. 2500 calories should be like cutting weight at that age(i am 19 as well)
Are you serious about the oatmeal?? How is that a cheat idk bc that is a staple food for me. It sounds like you need some carbs .
I would also look at trying to fix your training regimen if your only getting in there 2-3x a week and you can't be doing the same thing forever you gotta change it up.
So what do you wanna fix first and we can go from there?