Just started running again and every day now my left butt cheek twitches, no pain, last around 5 seconds each time. why could this be happening?

Mm fasiculations: You may have a simple muscle fasiculation, which is common. They are the "twitches" that can occur in your muscles as described by you. They are generally benign and can occur in overused muscles, possible minor fluctuations in electrolytes (such as potassium), of when a nerve is irritated. They usually go away on their own, or massaging the area can help. Try stretching before and hydrating.