Very strong uterine (?) pain approx once a month (mid cycle) my periods are getting longer, my cramps wake me up at night, but bearable during day.

Difficult to say. Some women experience pain with ovulation. A normal menses is less than 8 days. Menstrual cramps can occur at night. However, since your symptoms are new, infection or another abnormality is possible, so I believe you should schedule an appointment with your gynecologist to be seen.