Pain After Sex

So I’m hoping someone else feels this so I feel like I’m not alone here. I’m 18 weeks pregnant with twins and about a couple weeks ago I started feeling very bad pain after sex. During sex, there is no pain and after sex if I am laying down there is no pain. However, if I stand up for more then like 2 min after sex within like 6-8 hrs I feel so much pressure. It feels like my vagina weighs 1000 lb! That’s the only way I can describe it. But it goes away the second I lay down again. My vagina def swells up after sex so I am not sure if that has to do with it or not. I’m gonna ask my dr next week but just wanted to see if anyone is dealing with this same issue and see why it’s happening. It seems like the only time I can have sex is right before bed.