Ok I had some pain in my abdominals. Help...?

Ok so recently I have noticed that I have gain some weight. And this may be just because I'm growing...Not really. Or because I eat too much. But, Ok. So I went running two nights in a row. And I ran like a mile both nights. And then the second night I was like Ok, the running is probably going to help my thighs, but I also want to flatten my stomach. Ok so after my run I went and I did 150 sit-ups. So, that was really stressful on my stomach. And the physical kind of stress. So then the next day, I had like terrible pain in my ab's. So like everytime I lifted up my arm, the area under my armpits would hurt and like anything that required me to move my stomach, it hurts. The pain lasted for only 2 days, and doesn't hurt anymore. But, I just wanted to know if I should continue to do the sit-ups. Maybe the pain is like from burning calories. Or something. So if you guys could help me out that would be wonderful! Thank you guys! OK.