I recently put sliced Strawberries and Blueberries in a tupperware, covered with sugar, and let it set for a day (in fridge of course), so the sugar may draw out juices from the fruit. The plan was to let it set for 2 days, once it was completely liquidized, heat over a pan to turn into a syrup for oatmeala, smoothies and such.

It's been about a week now, haven't had the chance to heat over a pan due to my work schedule, and I came back to find there was enough pressure created on the inside to make a corner of the lid pop off, meaning the fruit syrup was creating gases of some sort. Further evidenced by the center of the lid bulging out, meaning some some of pressure was created. Does this mean it's in the fermentation process, or creating something that could cause foodborne illness, or is it still safe to use?