is my puppy sick?

Question
Bella is my 3.4 pound, 12 week old Chihuhua puppy. I got her at 7 weeks. She started her shots at 8 weeks and has gone every 2 weeks since. I take her to doggy daycare Mon-Fri while I'm at work. She started going three days after i got her and the vet is very impressed at how well socialized she is. I'm 24 and work full time. They keep her secluded in a cage in a room that other dogs have never been in.(the cat room). They do this because they don't want to risk her catching anything. Today they called me and told me that she was not acting right.She was just laying around acting depressed. I left work immediately and rushed there. She sat up in her cage and waged her tail and licked me when I picked her up but she usually is jumping all over and she wont leave my side ever. She's a typical puppy, she is never still unless sleeping and she plays A LOT! I took her to the vet and they checked her temp. and listened to her heart. They said that it all seemed fine and I should just keep an eye on her and bring her back with any other symptoms. She is back at daycare right now and they are going to keep a close eye on her for me. I keep a constant eye on her but sometimes she sneaks away from me at the house. My mom is dying from cancer so Hospice has left all kinds of medications and things at my house. We obviously keep them away from the dogs (I have two others) But I'm afraid maybe she got a pill or something. The vet. did say that it most likely would have caused an instant problem since Bella only weighs almost 4 pounds. I'm just so worried. I know it's probably nothing. I had a rat terrier that my neighbor fed rat poison to and I took her to the vet once a week to have her system flushed out and spent over $5,000 trying to make her better before they said she was in pain and I had to put her down. I love my puppy to death and I'm so afraid something is wrong. Should I have the vet just start running tests or wait another day and see if she gets better? She is spoiled quite a bit and she's like my baby. She helps me cope with everything I"m going through with my mom right now.

Answer
This is NOT "nothing" and your small dog does NOT BELONG in any "day care" that has her caged all day!! GET HER OUT OF THERE ASAP.

Find another veterinarian as fast as you can.  At 4 pounds and 12 weeks, your dog could die of hypoglycemia faster than anyone would notice.  She requires a steady supply, in small amounts, of food and water and an emergency supply of glucose containing food stuff for any apparent attack of hypoglycemia.  NO VETERINARIAN WHO KNOWS WHAT S/HE IS DOING WOULD GIVE AN 8 WEEK OLD, FOUR POUND PUPPY VACCINATIONS AT TWO WEEK INTERVALS.  You are with the WRONG VETERINARIAN.  FIND ANOTHER ONE and get that puppy OUT of that day care situation.  Hire a pet sitter to come to your home 4x a day to hydrate your dog and observe her.  A tiny dog like this CANNOT be left alone all day.