Intuition or Overthinking?

I'm dating this girl for almost 3 years and we're great, not until her past flings messaged me, she asked for forgiveness and we're okay.... now I feel uneasy and feels like she's doing something fishy (we're in a LDR) and she behaves weird also, she turns her dnd at night and now I can't contact her and it's been days and I've seen a pattern. Guess I need to focus on myself again, I'm mentally ill btw.