Need advice please

So I’m pretty inexperienced and this is probably a very dumb question, but I just need reassurance. First off, I have the nexplanon implant but I’m paranoid so I don’t completely trust it alone. I started my period today and my boyfriend and I were messing around some. We were in the shower, and I had a tampon in, but I was giving him a blow job and came on my chest. Could some of his semen have washed down my body and gotten to my vagina. Like I said I had a tampon in, so that would have blocked everything right? And would the sperm have died when water hit I’m just very new to this stuff and I’m nervous to ask him or a friend cus it’s probably so dumb that it’s embarrassing. Can I get pregnant that way even if I wasn’t using a tampon? His penis never actually went anywhere near my vagina but I’m just scared it could have dropped down and some how made it into my vagina and past the tampon. Is that even possible