Hi and welcome to the site,
I'm sorry that you have this worrying situation. It sounds like you were examined very thoroughly. I'm glad that nothing serious was found.
As a pregnancy
advances, there is a lot of pressure and fluid that builds up in the pelvis and genital area. This can cause some of the normal structures around the opening of the vagina to swell up and hang down. At the opening of the vagina there can be several "hymenal remnants". This is tissue that is broken open when a woman loses her virginity. Sometimes these pieces of tissue are not noticeable. Then during the later stages of pregnancy, the tissue can fill with fluid and stick out. The tissue sticking out can get red and irritated.
I hope that there isn't anything serious going on. But if you've already been checked completely, it's not likely to be anything concerning.
I hope this information was helpful.