Thanks to both of us!
If anyone is interested the issue/bug that Goobles encountered is due to handling the collision when the character intersects a solid tile, and it is beneath it. What I did was to set the y velocity to 0, but as it caused trouble I'm trying to make a better solution.
Also, I'm working on polishing the code, and looking into pointers to functions, to methods, etc, so I can make some buttons classes and make it more appealing to the look.
Last thing: did any of you noticed that you can look whats below you by holding the down arrow key? I didn't put that on the Help>controls, as I thought that might be some "instictive thing", like I press down so I look down.