You may have a uterus that clamps down one baby and then because of many factors may not the next. Those factors include interventions as well as nutrition.
#1- They need to be fed. Protein feeds muscles, as well as other things that supply good nutrients to your muscles. Iron is a big one of these factors as well as calcium and magnesium.
#2- They need to be exercised and not over the top exercised or under exercised. This is the balance that is hard to find. Often artificial oxytocin will hyper-stimulate a uterus, which makes it unable to clamp down. This can also happen naturally too. A uterus after a hard labor, too many herbs someone took or other things can cause a uterus to be hypertonic. That is why something that is gentle can sometimes be recommended by doctors, midwives etc. to help uteruses stay healthy. Also, a uterus that stretched out, then never has had the chance to reduce in size, heal and begins stretching out again is more likely to struggle. (This would likely be the worry for you, Laura, since your babies are close in age).
#3- Uteruses need support after birth; they look like a heavy ball of muscle, hanging limply on ligaments in your pelvic area. When you are lying down, those ligaments tighten; the contractions of the uterine walls tighten and reduce it in size. When after birth, we stand for long periods of time, especially the first two weeks, sometimes we stretch out those ligaments, cause the uterus to not be able to go back to it's original size and struggle all the way around. Then if you get pregnant in the first 5-6 months PP, before it has had a 9-month healing time, you uterus can struggle.
Something else is hormone level. This can affect things with the uterus as well. If your hormone levels are not right you may not produce normal amounts of oxytocin on your own, or sometimes the baby is struggling to produce it.
Another big one is not your uterus as well, but your pelvic floor. If your pelvic floor is weak, when your uterus contracts, it sort of has a smooth pushing down motion and pushes against the pelvic floor.
I think though, sometimes you can do all you can, and some people just have a weaker uterine muscle than others. That is something you have to be thankful that we have artificial medications to force it to comply.
So, what is a good calcium and magnesium supplement? It differs for everyone. One that I find easy to take and many clients enjoy as well is Lifetime Liquid calcium. I prefer the strawberry flavor, but it comes in a variety of flavors. Vitacost has it for a fairly reasonable cost. Here is a link that as a referral will give you $10 off your first order! https://www.vitacostrewards.com/jTSZGtY Here is a direct link to the calcium Liquid Calcium and magnesium- Strawberry
Where can you go to find pelvic floor exercises? That varies, but there are some online resources available. This website offers some great exercise routines. I have not researched to see what they have available for pelvic floor exercises yet though. http://fit2b.us
These websites talk about for helping with bladder control issues, but it is the same muscles that help support the pelvic floor. They are easy and cheap to do. https://patienteducation.osumc.edu/Documents/IncontinenceExerProg.pdf
And another one.... http://mydoctor.kaiserpermanente.org/ncal/provider/juliealmeria/resources/dc/article?article=article_506667.xml&contentTitle=Ball%20and%20Band%20Exercises%20for%20Urinary%20Incontinence I hope these resources are helpful for you!