I am very regemented with meals during the week and the only difference yesterday was i ate lunch an hour later than usual, and it was a rest day from the gym so if anything less exercise, so thats why i am so confused.
As a one-off, I'd be inclined to shrug my shoulders and get on with my life, but in your shoes I might also do a little speculating.
We know our bodies strive for a sort of routine, and most of us found, initially, that our bodies would try to cling onto our higher numbers, because that was, to our bodies, our comfort zone. For some exercise increases bloods and for some it reduces them. As a gross rule of thumb, gentle exercise can tend to lower the bloods, but with more strenuous (gentle and strenuous are individual metrics) exercise, the bloods can be elevated, then fall.
I wonder if your body was still expecting the exercise, but wasn't subsequently stimulated for the energy (liver) dump to raise the sugar, prior to the post meal drop, so just dropped.
Either way, you may never know. You could repeat yesterday after a few days, just to see what happened, but your body could react the same or differently; hence my shrug and move along statement.
If you're not on hefty drugs, I'd just have a cup of tea and see how I felt. If you are on stronger drugs, known to drive your blood scores down, you might need to eat something. I don't usually consider Metformin as a hefty drug, due to the way it works.
We're all different, but certainly our bodies sometimes throw us a curved ball, just to keep us on our toes.