I had a normal menstrual cycle and a 1.5 weeks after I started a new cycle but with dark brownish blood but with no pain. Should I be concerned?

Irregular periods. Irregular periods are one of the most common problems obgyns treat. If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Infections, hormonal changes, polyps and fibroids can cause spotting. If concerned for pregnancy then do a home pregnancy test. There are many options available to help regulate the menstrual cycle. Schedule visit.