Could be several things.
What age are you ?
Sometimes when in the younger years, your Period, Duration can fluctuate. When younger I used to have 5-7 day periods. Usually flow heavy and with severe cramps for at least 4 days, then spot for another 2-3 days, with intermittent cycle, cramps and heavy flows.
As I got older my Periods adjusted to a more regular and dependable schedule,Of course after kids I had allot less cramping, as the Uterus had been stretched and the muscle was less tense.
In my late 30 and thru my 40's, I had 3 day periods, and rarely had any cramping at all. It was Nice to have Start to Finish 3 days total . At 48, I just stopped having them at all, I'm 56 now.
So your body can just be changing and adjusting.
How long has this been going on ?
Have you tried a Douche, like Summers Eve ? It may be that you don't " empty " all the way and residual Blood could be up there and the penis and your exercising the Uterine muscles may be releasing " trapped " blood.
The twinge of pain can also be the Re ~ awakening of the Muscles , so soon after the Period. This is why they recommend no sex for 6 weeks after birth, Giving time to get all of the Parts back in Place . lol
Are you on or have you just started a New Birth Control, pills or Device ?
This also can change periods and also pain, bloating, moods. Remember that your body was made to produce children and when taking something to prevent Children, you are " Messing with Nature ". So there will be " Side Effects".
Of course the " Side Effect of not messing with Nature " can result in at least 18 years, if not longer, of Pain and Mood Swings . And in between, once a month you'll have the Periods to Contend with on top of the Children LOL
So, if the pain is not Excruciating and the Bleeding isn't Extreme. Check your timetable for next months period. Try to exercise the muscles, (Google Kegels) start a few days before expected period.
Have the Period, maybe Cleanse with a Summer Eve type product, give it a couple of more days than you did this month for actual intercourse , Not saying No sex, just use other kinds of Sex .
Then see if things change or are easier for you . If they are not, or at anytime you get concerned go see a DR. to rule out any bleeding in lining or cysts or other issues.