Welcome to HCM
You have given blood for some investigation or blood donation
or some kind of treatment drug was given through it?
It is possible that you have developed thromboplalebitis where vein becomes like chord usualluy 3 weeks is sufficient time it should resolve but it may take some more time, locally applying heparin
gel after consulting a local doctor,even applying local cold
compress is useful,for pain you can take simple analgesic
may be used.
Please do not worry it will dissolve,
Please take care