Zithromax Z-Pak and allergic reaction

This is my second day w/ Z-Pak. Almost immediately after taking the first dose it felt like someone had punched me in the stomach. I had been feeling so bad though for 2 weeks + I just figured it was probably just from being sick anyway. Well, I had extremely vivid nightmares that woke me up several times last night, followed by an odd feeling of dread or anxiety. I didn't realize these were common side effects and just chalked it up to something I could've possibly seen on tv earlier in the day. Today since I've been awake I've had a terrible headache, and my stomach (while I haven't used the bathroom at all) still feels like I've been punched, only the pain is more severe. I didn't think any medicine could have side effects that worked this fast, which is what let me to even looking up this medication online. I never experience side effects from medicines and have never had an allergic reaction to almost anything I've taken. This is extremely weird for me. I'm having a terrible time trying to make it through the day, and the anxiety/fear/dread just seems to get worse by the hour. So far it hasn't really taken away my symptoms for bronchitis, but i'm hoping after a couple more days i'll feel better....

I'm taking Zpak for my illness. I took the first double dosage yesterday during school and it really messed with me. At one point, I felt like I was going to throw up, though most of the time I didn't feel like my normal self. I laughed at things that weren't funny and just had laughing attacks. I believe I saw things that didn't exist as well. I took another pill today and it left me feeling like someone was stabbing my stomach. However, right now I feel fine, just a little tired and hungry.

Is it possible that I'm having an allergic reaction or the Zpak is reating with other medication I'm taking, like Prozac? It makes me feel very off, but it's hard to describe.