You are taking BACTRIM which consists of 2 very active components. There is an automatic danger combining CERTAIN antibiotics and E if these antibiotics (Zithromax, Biaxin, etc) are metabolized by the liver using the same enzyme pathway used by E (P450 cytochrome). When this happens, you could end up with VERY high or possibly lethal levels of E since the liver is unable to break down and metabolize the MDMA (think of the enzyme pathway as being temporarily blocked or occuppied).
This is a theoretical but real concern. There are no documented cases of E and antibiotic fatalities, although an HIV protease inhibitor (Ritonavir) has been linked to the death of a man in england who supposedly took 2.5 hits of E. He died of an overdose and autopsy revealed blood levels ten times what should have been expected given the dose. Ritonavir uses the P450 Cytochrome pathway.
Also, there are many studies showing that when two drugs using the P450 cytochrome pathway are taken together, the expected levels of the drugs vary.
That's the bad news
..THE GOOD NEWS is that Bactrim is not metabolized by the P450 cytochrome pathway. This means that you will probably not experience unusually high levels of E by combining the two. However, Bactrim is a powerful antibiotic that has caused sudden allergic reactions on its own including a few fatalities (very rare occurrences)...
The bottom line is that you will probably not have any problems combining the two, but no one can say for sure. You just might want to err on the side of caution and wait until you complete your 10 day course before you roll.
Besides, Bactrim can make you feel weird by itself and could interefere with having a good time. (I personally have ignored my own advice and combined E with "simpler" antibiotics such as amoxicillin or augmentin)
If you are taking any prescription drugs and are thinking about rolling, ask your pharmacist if the prescription drugs are metabolized by the P450 cytochrome pathway.
If the answer is yes, exercise caution and think about waiting until you are off the prescription drugs before you take E.
Hope this helps..