Here in Mexico it goes without saying that if you get really sick you should see a doctor.  Here are your options.

Hotels/Resorts- Many larger hotels and resorts have a doctor on call or on site that you can visit.  Fees are typically less than $1000 pesos. 

Hospitals-Most private hospitals will charge less for an emergency room visit than an office visit in the US. Most basic visits will cost less than $1000 pesos, ie stitches, anaphlatic shock etc.  A overnight stay is also very affordable if necessary so don't fret, get better!

Pharmacies-If you have a simpler sickness, many of the pharmacies here have a free or low cost doctor (max $50 pesos) available onsite to diagnose you and write a prescription right there on the spot.  Many speak some English (a few  and with you pointing at what ails you, the solution will be quick. This is ideal for general infections, including upper respiratory infections, ear aches etc, anything that one would see a regular doctor back home for.  If you need an injection, you can do it yourself or the doctor or pharmacist will do it for a small fee if not free.   

Clinicas de Salud- Mexican states like Jalisco and Nayarit offer free medical assistance for everyone at their health clinics (Clinicas de Salud).  They are typically very busy but can provide prescriptions, read X-rays, diagnose and provide vaccinations, even provide some medications if in stock.  Yes, even for tourists and long term visitors.  However most people prefer the speed of the pharmacies and hospitals. 

Doctors offices-walking around on vacation you will see a variety general internists to specialists.  Feel free to try them.  They are excellent and much more affordable than private doctors back home. 

Dentists-Mexican dentists are excellent as well and provide dental services at a fraction of the cost of back home. Many U.S. citizens wait till they visit Mexico for dental work because of this.  People have had work done that return home to the U.S. and Canada say their home country doctors overwhelmingly approve of the work but do mention that some of the techniques for advanced work are older.  But for your good old extractions, fillings and root canals, veneers, implants and such, open wide!

 

Note that Mexico now requires a prescription for antibiotics.  

 

Diarrea/Montazuma's revenge?  You probably have food poisoning.  Vallarta's water is pretty safe so you picked it up from touching something or someone didn't wash their hands.  If you are doubled over and erupting from both ends, go see a doctor to prescribe something to kill that demono inside.  If you have just a healthy case of the runs you can try pepto or imodium. If those aren't very helpful, try Treda which is bit more powerful.  You can get it over the counter at the pharmacies (Farmacia) and sometimes at your hotel's shop.  Take 2 tabs the first 6 hours if your case is aggressive and then change it up to 1 tab every 6 hours.  Taking too much will plug one up.   Many locals say that coconut water (agua de coco) helps treat and prevent this condition as well.  You can find it at many convenience stores.  The best is from someone on the side of the street with a table of coconuts.  It's costs up to $30 pesos.  After you drink it, they will cut out the meat and can add lime and chili powder if you like.  That chili powder will help burn out those demons! 
Vallarta area hospitals:  By the marina and hotel zone you have San Javier, Amerimed and CMQ.  In old town there is a CMQ 4 streets up Basio Badillo from the ocean on the left hand side.  In Nuevo Vallarta and Flamingos there is a San Javier by Paradise Village and another on the south side of Mezcales.