How do small particles cross a cell membrane?

If they are small enough, they can just slip through and not have to go through any of the transport systems or head/tails.

There are a couple of answers.you could have an infection, a sti, std, or other problematic health issues.i would recommend going to the doctors, if not, take a shower and wash down there, then wait a couple of days, and if it’s still there, then is when you should go to the nearest or trusted gynecologist of your choice.