I had an umblical hernia that I had repaired, and then it reoccured. The second time I didn't even know I had it....nothing sticking out, until the doctor found it doing a manual exam.
The pain does come and go...if the tissue gets caught in the tear in the umblicus, it gets strangulated and causes the pain. But it can then slip back inside the tear and the pain goes away. I assume the same thing applies no matter the site of the hernia.
I think it's best that you called the doctor, and get it checked out right away, to rule out the possibility of anything serious.
Let us know how you're doing!