This commonly happens when your thyroid is failing. Have you been diagnosed with hashimoto's?
As your thyroid finds it harder and harder to keep up and provide your body with enough thyroid hormones, your TSH gets higher and higher to attempt to produce enough. (It stimulates your thyroid to produce hormones, thus thyroid stimulating hormone ie, tsh ). Since your thyroid levels are normal, I would say that's what it appears is happening. But you should certainly talk to your doctor 🙂