You don't say why you're taking it, though this is an anxiety forum. This drug is most often used for a couple kinds of nerve pain but is sometimes used for anxiety and seizures. When you take drugs that either target or mimic brain neurotransmitters you can't just stop them -- you have to taper off of them slowly if you want to quit and you don't want to just run out of your prescription. -- that's quitting abruptly. You also don't say what your dose is and whether you took more than usual because you missed the three days -- that's also a bad idea. You can't make up for lost days and don't need to. No way for anyone to know if the back spasms are at all related to the drug -- it's pretty common for humans to get back spasms and you might have done something to tweak it -- but if you think the drug is responsible, you should talk to the doctor who is prescribing this for you. If you can't reach your doc quickly, you can call your pharmacy and ask the pharmacist if this is a side effect of whatever you're doing.