Sesamoiditis is effectively a bruising of the small bones under the big toe joint.
It might feel like you are walking on a stone, you might notice bruising and will get worse the longer you are walking / running.
It is allso very common in dancers as well as rugby / football players. It can be quite debilitating, but is very easily managed with semi-bespoke insoles as well as a variety of treatment regimes.
Physiotherapy can be helpful, however I would strongly recommend that you also see a podiatrist. The services offered by physiotherapy will assist in the management, but insoles are necessary to get on top of the initial pain you have.
There are two pea-sized bones under the big toe joint and these can become bruised when levels of walking / running are increased or with a bad landing.