My girlfriend woke up with the bottom of her tongue swollen and hurting badly. What could this be symptoms of?

Many possibilities. Could be from a dental infection, could be from a salivary gland infection, could be a small stone trying to get out. She needs to see a doctor right away as infections in the floor of the mouth can be serious and spread.
Many things. Could be a trauma, salivary stone, infection, cyst or tumor.--in that order of probability. Need history. What happened last night? Traumatic kissing/oral sex common. Pain w/ stones gets worse after eating. If not obvious, or no resolution in 2-3 days, see dentist for evaluation.