Yes, Zoloft enhances the action of the diazepam so usually the medications are monitored closely when given in this way. What dose was the diaz?

ETA: The drugs can be used together- it is just they are known to cause these effects especially with someone who is sensitive to reactions like this. If you found the diaz benifitial then maybe you can consider discussing lowering your zoloft dose with your doctor- but I suppose then you risk throwing out the balance you have established with the zoloft IYGWIM.
There are other anti anxieties you can try too...