Nausea, diareah once, right side pain that feels like muscle spasms 35 weeks pregnant! What could this be?

Many things. You should be calling your obgyn with this problem immediately so you can be evaluated, this is not a question to be answered on this web site, it may or may not be serious but it should be evauated.
Appendicitis, gastro. Gastroenteritis (mesenteric lymphadenitis) and acute appendicitis could cause these symptoms, however appendicitis would also cause anorexia/no appetite. Seek medical care if these symptoms persist and/or do not improve within 12 hours! differential diagnosis includes urolithiasia/colic, right adnexal torsion, and many other less likely culprits.