Hi, I'd like to help you with your question.
I need to ask some questions for clarification.
Are you having any other symptoms?
When did this start?
It sounds like your problem may be more related to anxiety than any serious diagnosis. I say this because you are able to forget about your intrusive thoughts for a period of time before you remember them and they come back. For someone who has a more serious issue, that is not possible. They usually live with the symptoms and are not able to forget about them at all.
If it would make you feel better, consider seeing a therapist for an evaluation. You can find one through your doctor or you can search on line at http://therapists.psychologytoday.com/rms/. It can put your mind at ease knowing that there is nothing more serious going on. Also, you may want to see the therapist briefly to work on your anxiety. Though it is not uncommon to over think things and worry, if you find that it is interfering in your life, short term therapy can make you feel better.
I hope this has helped you,Kate