Caution is advised when taking Midol with Pepto-Bismol. This interaction is regarding the long term use of aspirin, found in Pepto-Bismol, and acetaminophen, found in Midol. The combination could increase the risk of analgesic associated nephropathy. For your situation however, you are not planning to be taking these medications long term, and therefore there is a very low risk of any problems occurring with them.


Nevertheless, there is an alternative medication to treat your diarrhea symptoms that is available over the counter and will not interact with Midol. Consider taking Imodium Multi-Symptom Relief.


Imodium Multi-Symptom Relief is a combination product that contain the drugs loperamide and simethicone. Loperamide is used to relieve diarrhea while simethicone is used to relieve gas and bloating. Common reactions include constipation, dizziness, drowsiness, ad nausea.


Pepto-Bismol

Pepto-Bismol is a bismuth salt, specifically bismuth subsalicylate. While not technically aspirin, which is salicylic acid, it is a derivative of it and closely related. It is used to treat diarrhea, nausea, upset stomach, and H.pylori infections. Common reactions may include black stool, black hairy tongue, constipation, and tinnitus.


Midol

Midol is a combination medication commonly used to reduce pain and bloating. The medications acetaminophen and pamabrom are found in Midol. Common reactions while taking Midol include anxiety, increased blood pressure, constipation or diarrhea, dizziness, headache, insomnia, and photosensitivity. 


It is important to remain hydrated when experiencing diarrhea, especially during menstruation. If you have diarrhea for over 48 hours, contact your doctor as you may need an antibiotic.Only take over the medications as directed on the package and as needed to treat symptoms. 


Taking Pepto-Bismol With Midol

As mentioned at the beginning of this answer, there is risk of analgesic associated nephropathy with long term use of aspirin (and its derivatives) and acetaminophen, which are found in Midol and Pepto-Bismol. However, short term use is extremely unlikely to cause any problems. If you wish to avoid combined use of analgesics, consider alternative products such as Imodium Multi-Symptom Relief.