OP. You should take your rivaroxaban with food. This is because it might not be absorbed properly if you take it on an empty stomach.
Medicines are labelled with instructions if the instructions are important. If it doesn’t matter when they are taken it just won’t say anything.
If you have DVT symptoms again it really doesn’t matter whether or not you’ve been taking the rivaroxaban properly. You need to be seen again. You don’t even need to tell them that you might not have always taken it with food. Do say if they ask, but the really important piece of information is that you may have another DVT.
They’ll want to see you. Please go back to A&E or phone 111. Just say you’re being treated for a previous DVT and the symptoms have come back.