What Causes Irregular Period After Delivery?

I m 28 years old and for the past 10 years my periods have always been regular. 28-30 days apart. I m not on birth control and haven t been for years. I have three kids. My last baby is 4 months old. The delivery was normal and after delivery I was having no periods for three months. Now from the last 40 days my periods started irregular (trice in 40 days). I visited a gynecologist two times. At the first meeting she advised the following medicine; 1.     Transamin capsules (250mg) (Tranexamic Acid) 2.     Diane-35 (Cyproterone Acetate) Just used the above medicine for some days but condition was the same so I visited the gynecologist again he she advised the following medicine again; 1.     Transamin (250 mg/5 mL) Injection (Tranexamic acid) 2.     Brino 100mg (Tranexamic Acid BP) 3.     Orgametril (lynestrenol) The only issue that has been seen in the ultra sound report is that endometrium is thick. The issue now my bleeding continued intermittently from two weeks. Please advise. Thank you, Marjan