Potential back problems - Should I go to another vet for a second opinion?

Species: dog

Age: 4

Sex/Neuter status: female/spayed

Breed: shih tzu

Body weight: 11lbs

History: No history of any medical issues, up to date on shots

Clinical signs: lethargic, panting, whining when picked up

Duration: 1 week

Your general location: Chicago, IL

Last Saturday, my normally very energetic dog was laying around all day. She usually jumps up on the bed with me but she wouldn't do it, she wouldn't go up the stairs, she also knows a few tricks she wouldn't do, all of which involved her standing on her back legs. On Sunday night, things got a little worse. Not only was she lethargic and not willing to jump up on her hind legs, but she was giving very loud cries of pain when picked up. She was still walking fine, eating and drinking fine, and going to the bathroom without any issues. I took her to the vet Monday morning.

After an examination and blood tests the vet basically said he didn't know what it was exactly, but he was pretty certain the pain was coming from her back. He said it could be a lot of things from just a minor muscle strain or something more serious like a herniated disc. He said he wasn't concerned because her back legs were still working fine and he did this thing where he flipped her back paw to see if she would fix it and she did immediately. He said he didn't think an x-ray was necessary since he ruled out serious internal problems with a blood test and she reacted a bit when he would press on her lower back, he went down the length of her spine and she reacted every time he pressed at on specific spot. He prescribed a steroid and muscle relaxer, said I should probably not take her on walks for a week or so, and sent us on our way.

I've been carrying her up and down the stairs since I don't want her doing four flights of stairs multiple times a day anymore. She hasn't been on a walk (we used to walk 1-3 miles a day) all week. I only have a small studio apartment, but I've gated off a bit so she has less space so she's not walking as much. When I'm home, I make sure she's laying down as much as possible. I'm also giving her this vetri disk supplement even though I know supplements might not have any real merit, I figured it wouldn't hurt.

So far, she seems to be doing much better. She's got a lot more energy (toward the end of the muscle relaxers effectiveness). She's running and jumping even though I'm trying to keep her down. The only sign that she might be in pain is her breathing is short and rapid and her abdominal muscles are tight. She isn't whining when picked up anymore though.

My concern is that the problem might be on the more serious end of things and the vet wasn't strict enough about how much I should limit her - I read things saying up to 8 weeks of strict crate rest and that's nothing like what he recommended. On the other hand, I'm still limiting my very active dog quite a bit in comparison to her usual activities, am I being too strict because I'm freaking out over the worst case scenario? Could this just be a strained back and I'm panicking for no reason?

I know the internet isn't a vet, but all of the results when looking up shih tzus and back pain are showing IVDD, herniated discs, paralysis and other related horror stories. Should I treat her as if it's IVDD just in case and put her on strict crate rest for two months? Or should I go with the vets recommendation of just limited activity for a week or so? She does seem almost back to her usual self within a week, but I'm still worried there is potentially a bigger underlying problem and it's not being taken seriously enough. But the last vet visit just put me out $300 and I don't want to spend that kind of money again just because I'm being overly paranoid.

Any thoughts or advice?