Im guessing it's round ligament pain. I'm not sure though, I have had the same pressure for about 2 weeks now and my OB didn't seem concerned about it at all. (She did do a cervical check though because I also had pain with it, nothing was happening down there.)
I'm hoping your pressure goes away, it's so annoying to still have it coming and going.
I have had the same thing, often in the evening and it goes away if I lie down for a bit. I just asked my midwife if it was BH contractions and she thought not (but I'm 10 weeks behind you). She thinks it's just pressure and cramping from my uterus expanding and moving into place. So, I'd vote that, or BH contrax. Rest if you need to but I wouldn't be terribly worried unless its uncomfortably strong.