I don't know if you wear high heels a lot but it can cause like a tightening of the tendons because they are not stretched out from time to time. I had this and I had to wear lower heels and stretch multiple times a day. Just stretch the achilles tendon by pointing your toes towards you slowly 10-20 reps daily. also don't wear heels that dig into that area. Try wearing lower heeled shoes for awhile, you can also stretch like that @ work. Capasaicin cream (might be spelled wrong) is great, it burns at first but put it on for 6 weeks, my pain is gone for years!! my podiatrist recommended it and I never needed anything else with the combination of stretching also. good luck.
One possible explanation could be achilles tendinitis. I had achilles tendinitis when I was younger and I would feel the pain whenever I would play sports or be active. I know you said the pain is persistent, but if you're a very active person or have a job that requires you walking around a lot, achilles tendinitis is a possible answer. Regardless, if you want the best advice, see you doctor.
My achillies tendons are still enlarged but I have to wear shoes that don't irritate them.I was told that women that wear high heels get this problem with their tendons but I don't wear heels so I'm not sure why I got this.I would go to the doctor to see if your blood tests come back ok.
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