Help me please!!

So my last period recently finished and isn’t meant to be due for another 2 weeks. But since yesterday I’ve been bleeding and it’s really liquidy and red, I’ve had cramps on and off since then but I know it can’t be my period because it stopped on the 18th of April and it’s not due until 14th of May. I’m really worried as to what’s happening because this hasn’t happened to me before. I’ve also had sex with the same person since my last period aswell but due to my weight I’ve never been pregnant and I’ve not been on contraception, could it be that or am I overthinking it? What could this be because I don’t understand this. Also I had sex on on of my fertile days which was last Saturday