Have you done a test?
I would do one. Dry humping can still cause pregnancy as the material used is very thin and has tiny holes in it meaning sperm can go through. Sperm on the belly shouldn't cause any problems as it is away for the vaginal opening.
Just chill out for a while and do a test. If you really feel ill then speak to a doctor about it and they will help you. Being a teenager periods can often be all over the place so I wouldn't worry.
Take care and please
PM if there is any questions you have...