Most people who carry STD's do not even know about them because they have had no symptoms. This can be true of both HPV and Herpes. Being that you were with 20 girls, anyone of them could have given you an STD. Also since that STDs are so common, it is likely you could have more than one. You could have had this virus for months now and it is just showing itself now. There are many variables here and I would not pass blame onto your current girlfriend simply because you are with her now. In fact, it seems impossibly unlikely that the 20 former girls were all STD free. Do not be ashamed to go to a doctor, they see this stuff every single day. You should want to know immediately if you have anything and tell your current girlfriend if you do. If she were the one with blisters, you would want to know.
My Best,
Rainn
I would suggest going to a dermotologist to get it looked at.