Potential ACL re-tear. Lachman done while knee was swollen?

Long story short I had ACL reconstruction surgery 5 months ago and all was going well.

I recently noticed some swelling on my knee and told my physio. My physio examined the knee and said its very swollen. He then did the Lachman test and said "you're showing signs of a failed graft or re-tear".

Following this when he saw how distraught I was he said "it could just be the swelling and when it goes down we'll test it and it might be fine".

Is it likely that swelling could cause laxity and once it goes there isn't any?

I've not had any falls or injuries but I have been ill all week with some sort of virus / infection (nausea, headaches, sickness and diarrhea).

Is it possible for my illness to have caused knee swelling and the knee swelling to have caused positive result from lachman test?