I set a winning variation and the result was a positive percentage increase. I logged back in some time later and now the results are negative? Is this normal behaviour?

What's going on is that after you select a winner, data is still collected for that winning variation but is no longer collected for the losing variations. This means that if there is anything that causes an increase or decrease, it will only be reflected in the winning variation. Whereas, before you ended the experiment, changes would reflect in all variations.