I woke up two days ago with multiple painful ulcers in my mouth. they are mostly under my tongue and on top of tongue but can also feel in back or throat. what could cause this?

Viral? Herpes?: Several viral infection can do this. Depending on risk and exposure possibilities, this could be a first infection with oral herpes -- usually due to HSV1, but possibly HSV2 if you recently performed oral sex on an infected partner. Certain autoimmune conditions also are possible. Or just an unlucky large outbreak of canker sores. See a doctor ASAP, especially if you might be at risk for herpes.

Must see Dentist : It may be viral infection but try to rule out any abnormal oral sexual habits. You can also rule out if it did not happened after eating hot dog with mustered or after eating hot food like pizza which caused burns above and under the tongue . In the mean time rinse your mouth with Luke warm salt water and see if the blisters disappears in few days otherwise must see your Dentist .