Knotted, I'm actually a 130 mm in sit bone width, but I went with a 143 mm saddle. I figured it wouldn't hurt to have a little extra room as I change positions on the saddle. :)
I just (as in 1 hour ago) finished my certificate in pelvic rehab. (Physical therapy.) My whole class was darn near dancing out of the exam room! I don't think anyone failed the final exam.
Let me tell ya, I'm *full* of information right now! (dang, I hope I don't forget any of it later!)
First off, let's blame your saddle or saddle position, since you didn't have these issues before riding.
We won't blame *cycling*, cuz we all know cycling is faaaaabuuuulous!:D We won't blame core strength or posture, cuz I can't see those and evaluate them. (go to a PT who does bikes or women's health or BOTH to really get checked out. Forum info is never as good as face-to-face info. If you need resources or contacts in your area please PM me.)
Something is pressing on your soft tissues. What is it, and why is it pressing?
1. Is your saddle too narrow?
A narrow saddle might feel ok for a bit, but if you are weightbearing on your soft tissue or pubic rami or pelvic floor tendons rather than your ischial tuberosities ("sit bones") you will compress things that will protest. A well-fitting saddle should be as comfortable as a wooden chair.
2. Is your saddle too soft?
Soft saddles will also feel nice for a bit. Unfortunately, most wider aka "women's" saddles also seem to be very overly padded saddles. Cushy stuff will press upward between your sit bones as they sink down into the cush, and crush the life out of the soft tissue between the sitz.
3. Do you truly need a cut-out?
Ok, I've seen more female genitalia than I ever expected to when I began working in PT. And here's what I've learned: all women are built differently!! You might need a cut out, cuz your tissue might be "fluffier" than someone else's! Don't be shy! In the past I poo-pooed cut-outs because they don't work for me. Now I have seen the error of my ways. Lo, I must be a non-fluffy type. :D
4. Is your saddle/bar relationship wrong?
If you cannot maintain your weight on your ischial tuberosities on a saddle that is wide enough, you might want to look at how far forward ("reach") your bars are relative to the saddle or how far down relative to the saddle they are ("rise"). Reach and rise contribute to the anterior/posterior tilt of your pelvis on the saddle. Tilt has an awful lot to do with what tissues or bones are weightbearing.
Core strength is also vitally important, as is the tendency to lock your elbows (cuz then the jolts and bumps go straight to your butt) but you need face-to-face to work on that and other postural/riding habits.
BTW: elbow locking can also be a sign of too long a reach or too low a rise.
Saddle size: The rule of thumb (which seems to work well, so I have no reason to criticize it) is that you should take your outside sitz bone measurement and add 1-2cm to each end for wiggle room. That gives you the width of the saddles to consider.
Watch someone ride some time, they really do shift around on the saddle quite a bit as they change from downhill to climbing to flats.
Folks with a hip angle that feels best on "T" shaped saddles are at a disadvantage to those who can ride "pear" shaped saddles, I think. The "T" folks can't really shift front to back on their saddles like the "pear" folks. "T" riders just gotta get their buns up off the saddle more often to relieve/change pressure.