drops in fps after one match in dota 2

It could be that the second game was simply more demanding on your machine than first game, but if this happens regularly then it's more likely that your graphics card or CPU is getting warm and throttling a bit to avoid becoming too hot. Another possibility is that you're running low on memory and after a while the OS has to start paging and unpaging things to keep up with the game's demands. If you have more than 2GiB of memory then this probably isn't the case.