I would recommend a consultation with Dr. Heidi Grant. She's an Upper Cervical specialist in London. Here's her website:
http://www.nucca.co.uk/
Are you performing the stretches he gave you?
Here's a picture showing where the SI-joint is located - http://www.advpain.com/diagnosis/kbImages/SIJOINT.gif
Excess weight is a big contributor to low back pain. That's a likely culprit.
At this point, I cannot comment on the efficacy of your self-care regimen. If the pain hasn't subsided within 10 days, an exam seems warranted.
If funds are the primary reason you're not seeking care, you might consider going to one of the Logan College of Chiropractic clinics located throughout the STL area.
Based on your description, the sensations you're noticing are similar to those of axial nerve palsy. It's not uncommon to experience this after lower neck/shoulder injuries.
The problem often resolves on its own after a few months, but it's not a bad idea to meet with a chiropractor to receive an examination. PT is also an option.