What Causes Nausea And Vomiting After Having Cephalexin?

Question: My daughter was seen in the ER Wednesday for what we thought was a Sebaceous Cyst. They attempted to drain unsuccessfully but then gave Cephalexin 500mg qid & Motrin 800mg tid. She has taken Cephalexin before but not at such a high dose. On top of that she is on Prilosec & Carafate for hx of bleeding ulcers so we try to use Motrin sparingly. MD was aware of GI hx but gave Motrin anyway. Regardless today the site was worse, so we went back to ER where they finally determined if it had been a cyst it may have ruptured despite increase in size & apparent spreading of bruising. They added Bactrim DS & gave a dose prior to leaving hospital. She has never had Bactrim before. Prior to taking Bactrim she stated she was somewhat nauseated, but not enough to mention. After we got home, her nausea became worse & she just woke up feeling like skin burning (can't see rash yet) & she vomited. I have Zofran pills (maybe suppositories too) & Promethazine suppositories. Can I give her either of these for nausea & vomiting. Clearly we need to D/C All 3 meds to determine which is making her sick & go back to MD for medication adjustment as it hard to tell which is the culprit. I really need to know mostly about the anti-nausea options at this time