First, I want to express my appreciation that you did say up front who you are, and it shows real character that you decided to ask another group of adults for some insight into why another adult might have said something to you. Now, we can't tell you for sure why your mother is concerned about you running around the streets or whether your interpretation that she doesn't trust you is actually what she thinks. You will need to ask her yourself whether that IS what she thinks.
However, I can say that as a parent of a couple kids younger than you, I would NOT want my kid just hanging out for the whole summer without any planned activities. First of all, he would get bored REAL fast, or he would fill the time with screens. If he was just hanging out with other kids, they might just hang out... or they might slip into doing some less-than-healthy activities due to boredom. Second, summer is a great time for kids to do things which they don't get to do during the more-scheduled school year. It would be a wasted opportunity, perhaps, for you to just hang out when you could be exploring some different interests. Some downtime is important, however months of it is not a good idea.
Some other answers had a great suggestion. If you don't like the activities she has picked out for you, then do some research and find others which you know you like or you are curious about. Make a list with the costs, dates, and so on. You can build in some unscheduled 'hanging out time' into the summer if you really want that. Then schedule a time when you can have a good, relaxed conversation and present those ideas to your mother. It would be wise to be prepared to compromise, namely that you might do some of the activities she has proposed and some that you have proposed. Good luck with it, and I hope you end up with some great plans!