The link below advises on when to seek help.
It also mentions a pulse oximeter. This is a small device that measures your oxygen levels.
You can purchase one from sites such as amazon and usually costs less than £15.
Even if you do not feel breathless, your oxygen levels may be low. Please follow the guidance in this leaflet on where to seek advice.

It is important to seek help if you feel more unwell.
If your condition worsens, do not wait but act immediately.