With metformin not everyone will necessarily have any side effects. I found that I was OK for most of the time but I just had two or three bouts of the runs when I ate more carbs in those few meals than I had been. So my advice is to watch your carbohydrate intake - which you probably should be doing anyway.
My doctor also advised me to cut my dose down if I did experience any side effects, then wait a week and gradually up the tablets again, by one a week (I was on three a day), so that my body could acclimatize more slowly to the tablets. I didn't have any other issues until I was given a different brand a couple of months later, which gave me bad nausea and headaches, and our pharmacist then changed me to a slow release metformin, and made a note on my records not to give me the brand that had caused these issues.
I've had no experience of invokana so can't help there.
Robbity