Thank you for th additional information.
The pain from flexor hallucis longus tendinitis can occur anywhere along the course of the tendon along the bottom of the foot, the ankle, and up the back of the leg.
The lack of other medical problems or medicines is the more important information, to be certain that there was no reason to not take any recommended medicines.
There are several interventions
for flexor hallucis longus tendinitis. The usual medicine is an anti-inflammatory medicine, and two are available over the counter - ibuprofen and naproxen. However, in someone with a more severe inflammatory problem, it may be necessary to take a higher dose, up to 2400 mg per day of the ibuprofen or 1500 mg per day of naproxen, in divided doses. There are prescription strengths of 800 mg of ibuprofen and 500 mg of naproxen, designed to be taken 2-3 times per day, but it is also OK to take an equivalent amount of the over the counter strengths.
Other interventions that are also used for conservative treatment of flexor hallucis longus tendinitis include rest (particularly avoiding exercises that stress the tendon, such as jogging) and immobilization when sleeping. Immobilization can be done using a splint or removable cast that supports the entire foot and ankle in a neutral position (i.e., bent neither up or down) that can be placed on the foot and ankle during sleep. It is also appropriate to perform stretching exercises, such as while standing facing the wall, place a book under the large toe and then bend at the knee to flex the ankle and stretch the tendon. Once the tendon is maximally stretched, the position is held for 10 seconds, before standing. This is repeated for 10 repetitions; a set of 10 repetitions is repeated 3-4 times daily.
If the conservative management does not work, then it would be appropriate to be seen.