Kittens dying at 7 weeks old please help

My queenie had her kittens 7 weeks ago. Everything has been perfect, and she had 5, and all 5 had survived. About two days ago, we found out favorite one dead in our bathroom. We had no idea what happened. Then later on that day we found another one. And then today I found the third one laying on the floor, also passed away. I dont know what to do. I have two seemingly healthy ones left, but I am terrified I am going to turn around and find them as well. The other two are eating kitten chow and drinking water, and still nurse on their mom. I had put a flea collar on momma about 2 weeks ago, and recently found out how dangerous they are :0( and I feel so bad thinking I poisoned my babies. I took the collar off of the mom and wiped her down days ago, and also wiped down the remaining kittens. Should I be worried? Any one have any idea whats going on? The remaining two (Peanut, and Trouble) are very playful, and cuddly, and like I said eating and drinking wonderfully, but now I'm paranoid. Thank you to anyone who has any advice, its very appreciated.

My heart goes out to you, how horrible! It could be some kind of genetic problem, maybe with the heart, you might want to bring the remaining kittens in to be examined by a vet. If they were being poisoned somehow, you would think there would be some kind of symptoms, vomiting, lethargy, stop eating, etc., what ever it is it happens fast. Make sure you examine every nook and cranny for anything suspicious, remove houseplants to another room for now too. Were there any signs at all of something wrong? Distemper takes them one by one but they are really sick before hand and don't eat or drink for days. You don't have another animal in the house? Even a small dog can kill kittens quickly.  Were they going in the litterbox OK?You might consult with your vet on the flea collar, they might have chewed on it, or licked it. Vets are very experienced with all kinds of problems so I would talk with one for ideas. I know this doesn't bring your precious little ones back, but I'll pray that your remaining babies stay safe, please keep us posted on what is happening. RIP you precious babies, even though your time on this earth was way too short, you will always be remembered and will be held in a loving heart forever more. I pray your journey to the Rainbow Bridge is swift, you will be together always, playing your kitten games amongst the stars!

Thank you so much for your kind and heart warming words. I really appreciate them. The only other animal is another cat, and she pays them no attention. And no no signs of being sick at all until the day they died. I'm going to try to take them to the vet, the problem I have right now it that I'm out of work until the end of the month due to emergency surgery, and I have a 18 month old, so we dont really have extra at the moment. And my son wasnt near them, we kept them blocked off in the back half of the house for their safety. The whole thing is really depressing, they were all apart of our family.

Distemper can take kittens very quickly, even within 24 hours. They can be playing, eating and running around one day and dead the next. If there's any way to get the remaining kittens to a vet, please do so.