My gf has a tongue piercing that she's had for awhile, yesterday where the bottom ball meets the bottom of her mouth is irritated and has a small bump?

Localized Infection?: Because a piercing in the mouth could be equivalent to an open wound, there is a risk of infection. This can happen at anytime even if it hasn't been an issue in the past. Please have it checked out by her dentist or an oral surgeon to evaluate the severity.

Take it out: Tongue rings are terrible for your teeth and soft tissues in the mouth. The hard metal rubs on the teeth and wears them down, and rub on the soft tissues and irritate them. Could also be an allergic reaction to the metal. Tell her to take it out and let the area heal. If the irritation doesn't go away in 7-10 days, see a dentist.

Blocked Saliva Gland: Any piercings that communicate with the oral cavity can be very damaging to the hard (teeth, bone) and soft (gum, tongue, lip, cheek) tissues. While i don't like to see these piercings, i recommend plastic instead of metal as this is slightly less traumatic to the tissues. It could simply be irritation or could have caused something else like a blocked salivary gland. See a dentist for evaluation.