It's the ligament loosening, not the muscle, which is why it is important not to overstretch - keep legs together as if you're wearing a mini-skirt with no knickers, is what I was told. It works if you remember that. Keep weight evenly distributed, don't carry heavy loads. It is important not to overstretch because ligaments don't stretch and come back like muscles do, and the relaxin is making those important ligaments quite loose in preparation for the birth, where the ligaments will pull the pelvis wide to allow the baby through.
Feeling this close to the birth is normal. Feeling it weeks before you are officially term is SPD, where it has loosened too early. If you are in the latter category, see an osteo (physios will tell you there's nothing you can do except wear a brace and keep your legs together, but an osteo will give you a better treatment that will prevent your back muscles from tightening (thus, shortening, thus leading to lower back pain) as they compensate for the extra movement in that area), as soon as you can. It just makes the last bit of your pg unbearable at times.
For me, it felt like someone took to my pelvis with a sledgehammer, from underneath. Another way to describe it is like landing astride a big rock from a great height!
If it's not that severe and you are officially term (within 2 weeks either side of due date), then it's likely to be a short term affliction.
If you are BFing, and you have SPD, prepare for it to revisit later on!
I just decided that I was not prepared to be told "it goes with the territory" and to just, essentially, suck it up. I did lots of research and then got a second opinion. There may not be much you can do about the relaxin in your system, but there are effective management strategies that not all physios know about. If you DO prefer to see a physio over an osteo, then please, please get one that specialises in women's health (usually attached to hospitals).
I may not be a midwife, but it's something I have a clue about from having been exactly there in my last trimester and back again purely because I'm BFing still.