Thank you for your reply.
Well causes in such case can be DVT, cellulitis, injury, gout, osteoarthritis, skin infections, ruptured bakers cyst, venous insufficiency.
As your doppler scan is clear so DVT is less likely option here.
Presence of bruising, redness and tenderness point more towards cellulitis, injury, osteoarthritis, ruptured bakers cyst.
You need is to get X rays of the foot, blood tests to see for infection.
As you are diabetic so i would say in all of the above mentioned causes cellulitis is most likely in your cases. Diabetes can prone to person to infections as immune system is suppressed. This needs to be seen by your doctor as soon as possible as infections (cellulitis) can spread rapidly in such cases.
I hope this helps. Let me know if you have more questions. Have a nice day.