My tongue ring has started to hurt. what could be wrong?

Possible infection: You need to have the tissue around the tounge ring evaluated for a possible infection. Most likely you will need to take it out for awhile and possibly be put on antibiotics. See a dentist or physician to evaluate. You should probably do it soon so you don't end up in an er if your tounge swells to the point where it may hinder your breathing.

Infection: Tongue rings as well as any " jewlery" in the oral enviroment are prone to collect bacteria around them. The mouth contains thousands of good and bad bacteria which often collect around these artificial surfaces. Sometimes cleaning alone will not remedy this and they must be removed.

Infection likelly: You probably have some inflammation or more likely an infection. Sometime an infection is obvious, while at other times it is assumed or needs to be prevented from getting worse. I'm sure your dentist will place you on antibiotics regardless. This would be a great time to decide to remove it for good anyway, as it will be prone to re-infection and chipping teeth over time.