It absolutely could be the Zoloft - it could be something else as well. But if you stick around here very long (and I hope you do), you'll hear the mantra about never going off anti-depressants "cold turkey". Once you start taking them, you need to be sure you have a plan in place that will "guarantee" that you can get your medicine - or a way to go off of it under a doctor's care.
Pharmacies (at least mine) are usually good about giving you a single tablet to tide you over if for whatever reason you've run out (say you've lost your meds). They understand well the seriousness of going off these meds. But if you don't have an active prescription, you must contact your physician.
I'm confident that you'll be fine - but call you doc today and do this the right way.