I Have A Soft Lump On My Achilles Tendon Above

I have a soft lump on my Achilles tendon above the ankle, it carries on towards the inner ankle bone, it is slightly sore and aches slightly but doesn't bother me too much. I have had tendonitis in the past and mostly wear flat shoe. I work on a supermarket checkout and spend around 4 hours in a 'perched' position, then get a 30 minute break and then another 4 hours, I have had stiffness in my calf muscles since doing this job and experience pain in the lower legs at the back when I stand after a 4 hour stint. After I walk for a few minutes this eases. I'm a bit worried about the lump as I have had cancer (breast) twice.