So I've been picking up the occasional judo class after jiu-jitsu and for some reason we started working on de ashi harai. I'm still a white belt in judo (thinking about actually trying to rank up, study it deeper) and mostly work on ouchi-gari, tai-o-toshi, ippon seoi-nage, and o-soto-gari. No one ever shows up to judo class. Seriously I'm the only jiu-jitsu guy who goes, its weird. So its just the sensei (a 3rd dan, he is actually the assistant but super experienced and technical) and my buddy who just got his 2nd Dan. We just worked on de ashi harai over and over and over again. Literally maybe 45 minutes of me just working that throw, and only one one side. Really breaking down the mechanics of the throw, the kuzushi for it etc. That night my right hip flexor was insanely sore. I stretched, have been stretching it out, and took the next day off just to rest. It feels better, is still sore, but I wonder, is this "normal" judo soreness? I've never really gotten sore hip flexors before from training.
I've never had sore hips, but what you're describing is a pretty normal method of practice. I personally only really use Osoto Gari, Uchi Mata, Sumi Gaeshi, O Uchi Gari and Sasae Tsurikomi Ashi and I frequently spend the whole class drilling just one of them and trying to catch it every time in randori. It's like how bodybuilders have a leg day, back day, shoulders day, arms day - it's often better to put all your focus on one thing for a session than spread your focus around.
Take some anti inflammatory with some juice and take a nap.... your leg probably wasn't ready to endure 45min of foot sweeps. One the upside, your timing for it should be superb by now.
If you're attacking correctly with de ashi barai, you'll be using your hips, so not surprising that your hip flexors are sore.
It could also be from falls, depending on how many you take. You are falling on your hips every time.
