This is the funniest OP I've ever seen on a TT forum. It's not that easy to get heavy topspin on a pendulum serve on purpose, to do it by accident while trying to serve backspin takes genius.
Assuming by "I am serving regularly" you mean fh pendulum serves, the difference may be where in the paddle you are making contact, and at what point in the trajectory of the paddle. If you touch the ball with the tip of the paddle while the paddle is moving down towards the table (early in the swing in other words) you might get heavy backspin/sidespin. If you touch the ball while the paddle is moving up away from the table (late in the swing) you might get topspin/sidespin.
Since you aren't sure what spin you are making, just read it off your opponent's rubber. If he pushes and it pops up, topspin. If it stays low, backspin. If he flicks, then you already know the ball is coming to you with some degree of topspin.