You can continue to have side effects from interferon for months after stopping the meds. It alters your immune system and interferes with other systems and takes 6 months for the body to creep back to normal function. It can cause signs of alarming problems that are not really there, just systems acting up in confusion. Makes it really hard to tell what needs medical attention and what needs to be ignored and tolerated until more time has passed.
I was on IFN/riba for 7 months and a PI was added in for 6 of those 7. After 6 months on TX I developed all kinds of bizarre things, like OTC drug allergy, sun allergy and peripheral neuropathy (nerve damage that affects extremities). After TX stopped, I had a recurrence of peripheral neuropathy and developed terrible pain in one knee that came for a few days, then went and returned as it pleased. It took 10 weeks off of treament meds for all to sraighten out and for new, crazy things to stop popping up.
If there is a reason for you to have back pain (strain, bone spurs, degenerated discs, etc., you know your own body) get it checked out. An X-ray from a good orthopedist who knows how to read it is worth its weight in gold. If not, ride it out. It will straighten out when you have some more distance from the treatment meds.