Hey mama, lower back and pelvic pain is super common sa third trimester, a few of us went through that din kasi lumalaki ang baby at nag-shift ang center of gravity, pero since it started suddenly after your nap and it's one-sided on the right, I'd really suggest checking with your OB just to be safe, lalo na if it doesn't improve or gets worse. Could be ligament pain or positioning, pero better peace of mind lang talaga, no? In the meantime, try different sleeping positions, maybe a pregnancy pillow if you don't have one yet, at some gentle stretching, pero don't push it if it hurts. Keep an eye din if may other symptoms, you're already cleared for UTI so at least that's ruled out na, which is good! Let us know what your doc says, kasi first time moms really deserve to feel confident about these things.