Can't feel strings + weird period

I got my IUD (Paragard) a month ago today, so I'm due for my first period any time. I've been cramping for the last week but have not started bleeding. My partner and I ended up having sex last night, and I started bleeding almost immediately after. It was like period blood, not like something was wrong. I know sex can sometimes jumpstart your period, and I was relieved to see blood because I like verification I'm not pregnant. But today I'm not bleeding and I went to check for my strings and can't feel them :( my partner finished inside me, so I'm worried my IUD has moved and I'm at risk for getting pregnant. Just wanting to know if anyone else had something similar happen/is it normal to not feel your strings at certain times of the month?