what do I do if my young bunny has diarrhea?

I got two female bunnies a little over a week ago, one is a lionhead and the other is a holland lop... iirc, the lop was 8 weeks old and the lionhead was 7 weeks. today, I discovered that the holland lop bunny has (mild) diarrhea and I'm not sure what to do.

diet:

the breeder gave me a big bag of food pellets so that I could feed them the type/brand they were used to. at first, I was feeding them that and timothy hay (with both available 24/7), but they seemed to be getting chubby in a hurry so about three days ago I started restricting the amount of food pellets they were offered each day (so that they'd eat more hay/less pellets) while keeping hay readily available at all times. I also switched them over to alfalfa hay because I thought they might like it better. (I read that alfalfa is good for young bunnies as long as you switch them to timothy at 6-7 months.)

the one that has diarrhea seemed to eat almost entirely food pellets when I was offering timothy hay but has been eating a LOT of the alfalfa, so I'm wondering if the dietary change caused it. I haven't fed them any fresh foods (veggies, etc.) yet - only pellets and hay. I let them play outside for an hour or so at least once every day or two (supervised and in a bunny playpen) and I remember her nibbling on the grass a little bit last night, but she didn't eat very much of it.

behavior:

she's always been very calm/not very active (borderline lethargic)... she's still about as playful as she was before, if not a little moreso (she was actually standing up on her hind legs, hopping around, etc. a fair amount when I was watching her just now instead of just sitting around or eating). she's drinking water normally (I emptied the bottle and refilled it with fresh water a few hours ago, which I do about once a day).

environment:

I cleaned their cage out completely last night aside from scooping some bedding from their pee corner into the bunny litterbox (filled w/small animal litter) that I'm trying to train them to use. I didn't use any soaps or chemicals to clean the cage - I just sprayed it with a hose and then let it dry while they were playing in the yard. they were outside for about an hour and a half last night (after it cooled off a bit) - my neighbor likes to come over and chat/watch the bunnies with me. I don't offer them food while they're out in the yard, but I always attach their water bottle to the playpen in case they get thirsty.