Hulk -- who's had multiple back surgeries in the past -- says he must have aggravated his old injuries ... because the next morning he could "barely move" and had to call an ambulance.
Hulk tells TMZ the doctors believe he suffered a tear in one of his muscles -- but they still need to run some tests to be certain.
As for when he'll leave the hospital -- Hulk says he could be holed up there for another two days ... but at least he has his Twitter account to keep him busy. (TMZ)
