Unfortunately, sometimes meds just stop working for some of us and there are a number of theories out there as to why it happens. You need to speak with your prescribing doctor and let him/her know that you're having break-through anxiety attacks. With Zoloft, you can go up to the maximum of 200mg a day, but do NOT do this without talking with your doctor!!!!! I can't emphasize THAT loudly enough.
I wish I could have given you a more definitive answer, but it sounds like you need your meds tweaked and regardless of what I say, only your doctor is going to be able to help you out there. Just know that this is NOT unheard of...........it's actually quite common.
See your doctor as soon as you can and please TRY not to stress out about this. It's fixable.
Wishing you the best