As per the description given by you yes it seems you had superficial thrombophlebitis
which is an inflammation involving a vein. Now that swelling and redness
has subsided the hardness will go in due course of time and you need not worry too much about it. What you can do is a bit of hot fomentation to the affected area i.e. apply some local heat. You may fly as there is no apparent problem with that.
Getting an ultrasound
is not going to harm but usually superficial thrombophlebitis does not predispose to DVT.
Wishing you a speedy recovery