1st off: you were doing "oblique bends"?! dude...
next: On the hip flexor it sounds like you're just reinjuring it. The recommended protocol is high reps at low weights and working up starting just a few days after the injury and moving up. Check out the Starr Protocol. (it keeps being mentioned... and google finds the wkia page for it, but i've finally found the original post:
Starr Rehab Protocol original post by Rip
So yeah... do that. Same thing should work for your back too assuming it's a muscle... Took me about a month or 6 weeks to recover from a moderate "pull" in my lumbar doing something similar, but not as rigorous. Prob would have happened faster if i'd done it as written, i'm guessing. Also, try to foam-roll or otherwise get in there and "work" the hurt area.