Posterior Tibial Tendinitis is not a common tendonitis, but typically occurs in runners. The main symptoms are either/or swelling and pain just above or below the medial malleolus. This is an overuse injury so the cause is when the load over time in the tendon exceeds what the tendon can take, usually when not enough time is given for the tendon to adapt to changes in load. It is typically associated with ‘overpronation’ if the forces of pronation are high.
This rant on video from Podiatry Soapbox a good overview of the condition and how it should be treated.
The management is not that difficult provided common sense and a logical sequence of events is followed, yet have been a lot of questions in forums and Q & A websites (eg here, here and here) with a mixture of the quality of advice being given online.
One of the biggest problems with posterior tibial tendonitis is runners is that in many places they confuse it with posterior tibial tendon dysfunction when they are two totally different beasts.
Plantar fasciitis is common and like any other problem that is common there is so much snake oil that gets marketed for it and bad advice that gets given for it. Because of this, so many cases get mismanaged and what should be an easily managed condition become a chronic difficult to manage problem.
The classic symptoms of plantar fasciitis is pain under the heal that is generally worse when getting up from rest. If you have those symptoms, then there is a pretty good chance that this is what it is. However, there are a few uncommon problems that can produce the same symptoms, hence the need to getting the diagnosis right. There is nothing wrong with self-diagnosis or self management as the simple cases are easy to treat. However, you do need to be aware that not getting one of the more uncommon conditions diagnosed and treated early can lead to a lot of unnecessary pain and discomfort.
Plantar fasciitis is due to too much cumulative load in the plantar fascia beyond what the tissue can take. This means that the basic treatment has to be directed at reducing the load and increasing the ability of the tissue to take the load. Any other treatments are just directed at the symptoms and not at these basics make a lower probability of success over the long term. There is so much differing advice been given of differing quality that may or may not be based on the most recent available evidence.
Whatever is going on with your plantar fasciitis, get the right diagnosis first and get the treatment that the scientific evidence has been shown to work better than a placebo, rather than rely on anecdotes and poor quality non-evidence based interventions.