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?

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