lump on gum after filling

Over three months ago I had a deep cavity filled. A month or so after I noticed a small very hard painless lump on the gum below. I went to the dentist to have it evaluated and after taking an x-ray the dentist said it was "a mystery" (the x-ray was clear he said) and not to be worried about it. He gave me antibiotics to take in case it started to hurt. Just recently, the lump rose even closer to the surface and popped. There was quite a lot of pus and blood when this happened. It continued to drain for a few days. Now I have what looks like an ulcer on my gum and the area has become red and painful. I have taken the antibiotics (just finished them) but it isn't healing. I am very worried about this because my dentist doesn't seem to know what is happening to me. I live in a very small remote northern community and he is the only dentist here! Could this be serious enough for me to take time off work and make the long trip to see another dentist?