I am sorry about the embarrassment, but when you visibly act like you are trying to hide something from a significant other it makes them suspicious. When I was a teen I noticed that my mom didn’t usually listen that much to my much younger siblings complaining about me or something I had done unless I tried to defend myself. That taught me something about human behavior. If you want to hide something you need to not look like you are hiding something because that arouses interest. Regarding privacy from tracking ads - I don’t use Facebook marketplace at all. If I want to buy something, I usually buy from Amazon. Among other things, that prevents spillover and cross tracking. And Amazon is completely reputable but God only knows about Facebook Marketplace! Secondly you need to look into the privacy settings on Facebook and your phone. Personally I hate pop ups and tracking ads and go out of my way to avoid them - Including using a pop up blocker.

maybe be honest with her on whats going on with your body. I think she can understand how you would feel embarrassed

If you can't be honest with your girlfriend then she isn't right for you.

This way you're not holding the relationship^^ After 10 years together she'll be the one to order stuff like that for you ;)

Ditch that bitch. She's got trust issues. And you've got issues as well, I suggest working on yourself.