Pregnancy can occur whenever sperm (which is contained in semen) gets through the cervix and into the uterus, into a fallopian tube and there's a viable egg there to meet it. So, anything that increases the chance of this happening increases the likelihood of pregnancy. Decrease the chance of this happening and you decrease the chance of pregnancy.
Obviously the most efficient way for this to happen is when semen gets ejaculated directly into the vagina and viable sperm get up to the fallopian tube that contains a viable egg (a condition that only occurs a few days a month). Again, anything that reduces that also reduces the risk. For example, a condom prevents the semen from getting into the vagina, provided that it's used properly and is not broken. Pills prevent the female from releasing eggs so there's nothing there to fertilize, provided they're taken properly. (Forgetting the one day that an egg would release into the tube could result in pregnancy).
Sperm can live outside for up to 72 hours and the little buggers can travel great distances. So, in theory it's possible for your situation to result in pregnancy. But, let's look at the risk. You are on the pill so your risk is lowered. You didn't take it properly and your risk is raised. He didn't ejaculate into your vagina so the risk is lowered. He did ejaculate on your belly which is sort of close to your vagina, a higher risk than if it had been on your feet or shoulders. If the water was running it could have washed some semen away and down the drain. On the other hand it could have been washed right over your vulva and into your vagina.
So what's the answer to your question? Risk of pregnancy is not a simple thing. It depends on a whole bunch of different circumstances. I think your chances are very, very low but it's certainly not impossible. Do everything you can to reduce the risk, taking your pills the way your supposed to is your best defense.
PaulK9378 is quite right. Whenever there is live active sperm near the proximity of the vaginal opening, preganancy can, indeed, occur.