Well, the bad news is that our periods and hormones go through changes as we get older. Really, what my period is today is nothing like it was in my 20's, 30's and beyond. Your period will shift, hormones shift, duration of days shift and that is all normal and part of the process of our body changing as everyone's does. But . . . with that being said, who knows. You've had 3 other kids and know your body. Weird periods are often a way of knowing you may be in early pregnancy. You can get a positive 2.5 weeks after the last unprotected sex. Don't test before then if you are a frequent tester. I was on the TTC roller coaster for 4 years at one point. Whew, that's exhausting. Fun, but exhausting. Fun in that every month you are high that 'maybe' you are and then the not so fun part of low when you get a negative and know you have to go through another cycle.
One thing I did that really REALLY helped me know what was going on was temp check. I had a simple digital mouth thermometer that I kept by my bed. I took my temp every morning and wrote it down. It took very little time to see the natural pattern of the temp being at a certain spot on day one of my cycle and then slowly rising about a degree higher around ovulation time. Then it stays high until you are about to start your period again. Unless, your pregnant and then it never goes back down. I knew if my period was coming every time with this. The two times it did not drop back down were my two sons. :)) It's biological so an undeniable way to know what your body is doing. So, give that a try. Within a couple of months, you will see your own pattern of temps and be able to watch this and have some 'inside' knowledge while TTC.
Good luck and let us know how it goes. I'm happy to chat any time!