A very suspicious and worrisome bruise.....?

ok so about ehhhhh...... 3 or 4 months ago, my friends and i were doing a social studies project. in the video, i was to be shot. now, since real guns were not an option (i actually like to live) we decided on paint ball guns. so i had never been shot by one so i wanted to see what it was like (and to see if the 'assassin' was a good aim) we did a practice shooting. well they missed my stomach, which was properly stuffed with pillows, and hit right above my knee. so it looked pretty bad for a while, you know, how bruises are and it started to fade. but oddly, it shows back up every once in a while. mainly when i am taking a shower. i have no idea what is up with it but its kinda starting to worry me. so instead of going to the doctor and wasting who knows how much, i'm asking the internet..... good idea? eh we'll see. so any ideas on why this bruise wont go away or if its bad or whatever. i just need info por favor. so, thank y'all for reading my story and thanks for any info you can give me on this subject. thanks!!!