Ttc. 2dpo, cramp left side, diarrhea, nauseous, light spotting on and off. next day i was fine. what could have caused these symptoms?

Hard to know: We can not always know what caused this variety of symptoms. The most important feature is that they have resolved. It sounds like you had a mild intestinal illness that luckily was short lived. If the irregular spotting continues see your gynecologist. If the diarrhea and nausea return then seek evaluation by your primary care doctor.