muscle confusion

hello guys, i'm here in the hopes someone knowledgeable can riddle me this.

i'm a 28yo woman who practises karate twice a week. it involves a lot of leg work because we have a slighly intense 30 minute warm-up routine with squats, bycicles, donkey kicks (to the back and sides), jumps etc alongside flexibility exercises etc etc etc before a full hour of basically squatting and doing mostly everything while putting weight on my thighs all the time.

my problem started almost 2 years ago (i have been training for about 4 years). i don't know if it's because i switched to a job that requires me to be standing up 8h a day - which allowed me to train twice a week and not only once btw - BUT my thighs hurt soooo much during our training basically right from the start (like a sharp pain that only goes away a bit if i massage it). i barely can do like 5 squats? wtf?

i looked at an image (lol) and i suspect it's the big muscle in the front (rectus femoris).

people keep telling me the muscles might be weak but then why is the m******cker big?? if i flex my leg he is pretty visible and noticeable....it's not like i'm a bodybuilder or something but why the hell would my muscles "get weak" when they are starting to grow big???

if i go on vacation and don't exercise for like 2 weeks, the first training goes so smoothly and basically doesn't hurt at all and feel like i'm me again. now....i can barely do the things well and as much as i used to do them. i was pretty capable before :(

has someone dealt with this before? if you read through all of this thank you!!