B12 injections don't always work the same for everyone.

I'd really support those with methylcobalamin(active form) oral 5mg daily on an empty stomach. At the end of 3 months your level should be at least 400, and hopefully 1000 or more.

Also I'd try icing your back area instead of heat. If you do use a heating pad 1) do so no longer than 10-15minutes as long term use of heat stimulates nerves, 2) don't apply heat to any area where you have put any topical rubs/creams. Heat applied to topical products drives the ingredients into the body past the skin and they can become toxic. A young woman died not to long ago from heat + BenGay. You can alternate heat and ice. Do either with moderation only.

Did you get your Vit D tested? Fixing that can improve your resistance to colds and sinus infections. If your Vit D is low, your cramping/spasms could be due to errors in calcium levels. Vit D will improve calcium absorption from your food.
Using OTC Vit D3 instead of RX D2 is more efficient and faster.

A level for B12 of 150 is quite low. It can take time for you to repair from this.