Congratulations you just had a baby!
For the first couple of months after birthing, don't use a cup. Your cervix is closing, your body is healing, cups and tampons aren't vagina friendly items. For your lochia, and if you're unlucky enough to have your cycles return right away after birth - use cloth pads. (If you're in the market for Cloth Pads, you can't go past www.scarleteve.com.au our personal favorites, and simply fabulous pads)
Pregnancy and birth are important and unforgettable milestones of your life. While you are beginning a new chapter of your life, some things will return to give you a glimpse into your former life. Starting your cycles again is a timely reminder of the ebb and flow of womanhood. Welcome back to the red tent!
At the same time that you are rejoicing your life and body is somewhat turning back into normalcy, there are obviously things that have changed!
Giving birth stretches your muscles, and generally weakens your pelvic floor to some degree. For some women, there might not be any noticeable difference at all, and for other women, your vagina might feel looser, and naturally your old cup might not fit like it used to.
The great news is most cup manufacturers like (Luna cup, Meluna, and Divacup) cater for weaker pelvic floors with a slightly firmer and larger cup model. If you have a favourite cup that you're using, switching to the larger size might be all you need to do.
Or, you might have a fussy lady garden (haha yes, I'm sorry... I do sneak it in there) that no longer gets along with it's pre-pregnancy friends, and needs to get out and meet new cups. You may need to try a different model cup to find one that suits you in terms of size, squishiness, and pop open comfort.
There is something else to consider. The changes might not be permanent. It takes a woman's body a year to fully recover from pregnancy and birth according to most doctors (And frankly we say it takes a woman closer to THREE years to be anywhere NEAR recovered fully from childbirth).
So, whatever is happen now might not happen six months to a year from now, particularly if you're diligent with rebuilding your pelvic floor strength, and haven't had any birth injuries. So do not worry to much if your menstrual cups don't fit or leak. It might be just temporarily.
Learn to love your post natal body. If you can't get your cup to play nicely, try switching (even temporarily) to cloth pads.