You could make an appointment with your doctor for your situation, but as you've surmised, it's quite likely due to your breakup with your girlfriend. Don't push sex as something that you need to meet a certain quota but let things happen as your feelings get back on track after the breakup. Keep up with school, work, and any other interests, enjoying friends or making new ones. You are young and able to rise above any temporary setback. However, if you do see your doctor, he/she might recommend a counselor, if you would want to talk out the changes you've encountered.