Periods Heavy For 2 Days, Medium For 2 Days, Then Light For 1-2 Days

Please tell me is my period flow normal. I used to get 5-6 day periods, heavy for 2 days, medium for 2 days, then light for 1-2 days until it tapered off and ended. It was this way even when I was on birth control.

For the past few months now, I have been getting 3-4 day periods, where the first day is really just some spotting indicating my period is going to be starting. The second day is my heavy day. The third day is medium light. The 4th day is light. I'm still on birth control. I am 30 years old now.

Also last month, my period color was dark brown and I also had some black colored clotting during my heavy day. I know some blood clots are normal during menstruation, I had them all my life. But this blood clot was black. Again this month on my heavy day, I had black colored clot. It was a dime size clot. Size was not large. Is black color clotting anything to worry about? I do not see red blood these past two periods like I am used to seeing. I am seeing mostly brown and/or black.

And how can I increase my menstrual flow. I am worried that my endometrium lining is thinning and can cause a problem when I do try for children.