I agree with the previous poster. Getting tested right now can help you determine if you have Herpes or not. It won't hurt to be checked. There is not a way to tell how long someone has had HSV. So this maybe something she had after your relationship.
Bumps can be anything. You may not even have it as not every encounter will lead to a transmission. However, it is something that a blood test will help you sort this out. You want to request a Type Specific IgG blood test for this. They can look at the bump you have as well and see what they feel it is.
First- if you are not sure what the sore could be- go get it tested! Herpes vary from person to person. You can be infected with the virus, but it can lie dormant forever basically. You can break out quite often with a lot of sores, or break out with just one sore. You can have one break out your whole life. Really, it just varies. It is rather painful though and itchy. Having sex without a condom is risky regardless if you know you have something or not. You can spread it for sure without a condom, even if you have no symptoms. Honestly, go make an appointment with your doctor or planned parenthood. Talk to them and they will tell you want you want to know and you can for sure find out if it is herpes or not.