It is difficult to diagnose medical conditions over the internet but I can give you some suggestions on what to do next.
There are a few different things that can cause vaginal bleeding. It seems that your vaginal bleeding happens at specific times; during sex when condoms are used or at certain times in your menstrual cycle.
It’s difficult to know if it could be an irritation from the condoms/lube or something else that is causing the problem. Given that it has happened 4 times I would recommend that you see your health care provider about this. They would be able to look at your complete medical and sexual history to see what could be causing the problem. It always helps when someone has the complete history for example what medication or birth control are you taking, past physical trauma to the vagina (episiotomy) etc…
I am not sure what STI tests you had but it is always a good idea to make sure chlamydia or gonorrhea are not causing the problem with vaginal bleeding. Your healthcare provider could see what tests you had done in the past and do these if necessary.
Please leave a comment to let us know if this answers your question or if you need more information.
For other readers, please feel free to leave a comment, or let us know if this was helpful.