This doesn't worry me a single bit.

I'd say continue, if you have the time and possibility to rest. Check or BP every day at the same time and position (at least 10 min into resting position always sitting, or always laying for 10 min) - just as a precaution.

The difference btw 90 and 40 is 50 - this is your Pulse pressure - and 50 is very very good. I'd think you body had rid of some extra stuff and just needs lower pressure (systolic and diastolic, both together).

If the pulse pressure (Sys - Diast) gets lower than 30, then check in here again, so we can figure that out.

Also, when you are laying down it is generally going to be lower by some 20-30 points than in any other position.