What size needle are you using? It's pretty unlikely with an 8mm needle that you'd hit muscle in your stomach, unless it's in a super lean area.
As Milos said, it probably doesn't matter if you did, as long as you didn't hit a blood vessel (in which case you would've seen blood on the needle). Otherwise your pep will just get absorbed more rapidly to the bloodstream from the muscle.
A bump could also just be a lipoma which can happen if you inject into the same tissue over time.