I can all but guarantee that it's SI joint issues, especially with it being your 5th baby. Your pelvis can get more and more out of whack and gets looser and looser under the effects of relaxin with each subsequent pg. My symptoms were the same as yours, except it all started at like 12 weeks. I couldn't put my pants on without sitting down or else I was in tears.
A chiro is really what you need to fix the problem permanently, but I totally get not being able to commit to that just yet. But you really might consider it after the birth, I think some of my problems are things that already existed pre-this pg so if you are thinking of having more babies it will help you avoid it next time.
But, short of that, you can try extra magnesium as it will help muscle contractility, ice (no hot compresses, too close to baby). Do things that encourage pelvic stability and keeping it in line...no crossing your legs (can any of us even do that anymore?
), no yoga twists, etc. There's a stretch that works specifically on the joint, if you sit up straight in a chair and then gently pull the opposite knee toward your armpit you should feel it. Do it on both sides even if only one side is affected to keep it even. You can also do it lying down. There's a belt my chiro gave me to wear that kind of hugs the pelvis in. Not sure if you can get those elsewhere. You can take homeopathic arnica, and use Traumeel homeopathic ointment topically. Don't worry that you don't take meds...I do (tylenol), and it didn't help.
Sitting on a birth ball kind of naturally helps things fall back into line and it helped when mine was at its worst.
I think that's everything...google "sacro iliac joint pain pregnancy" or something like that and you'll find all kinds of stuff. I literally feel your pain as mine was SO BAD before. With chiropractic care it's really been totally manageable the last half of my pregnancy and I barely notice it. But I was actually limping for awhile and was terrified my kids were going to run off and I wouldn't be able to chase them. It was bad enough to put me in tears, so I know what you're going through. My midwife also mentioned that her clients who've had this issue at delivery end up in some weird pushing positions just because it's trickier to find something that's comfortable. One even chose like a straight up lithotomy with knees back and everything. Just a tidbit for you!