Would pain under the bony knob of the inside of the left ankle and pain in the bend of the left ankle/foot be from posterior tibial tendonitis?

It could be...: If you suspect that may be the problem, (and it very well may be) use supportive insoles like spenco total support [http://www.Spenco.Com/products/footcare/total-support-insoles] and try on otc anti-inflammatory like aleve (naproxen). If this helps, see a podiatrist for rx orthotics and you should be good to go. If not, see a podiatrist for an exam and proper diagnosis.

PT tendonitis: It certainly can be. A sonogram or MRI can detect tendonitis or other tendon pathology.