I'm not a doctor but here's my uneducated, diagnosis-over-the-internet guess: you over did it. Muscles were probably cut during your back surgery and they weren't allowed to heal adequately before you started pounding those hills. Lack of muscle pain does not mean post-surgery healing is complete. That takes time (6 weeks?). Listen to your body for things like swelling around the incision or pain and listen to your doctor. Your idea of cycling is probably a lot different than your doctor's, so don't make biking a yes/no question with him. Ask him in terms of when, how much and how hard. Good luck with the healing.
'84 Trek 660 , '88 Trek 1500 ,'89 Trek 400 , '97 Trek 720 , '99 Trek 2200, '02 Trek 520