Sliding on corners on the bike

That's pretty rare unless:

* the pavement was wet (especially if there were paint stripes or metal grates in the road).
* There was sand on the road.
* you hit a pedal on the pavement in the corner.

When taking a corner at speed, you need to have the outside pedal down HARD holding the bike down.

Are you sure you didn't clip your pedal on the ground when leaning into the turn and pedaling? Much more likely than your rear wheel actually sliding/skidding out unless there was stand/gravel/wetness as others have pointed out.

If you're turning too fast, you're far more likely to lay the bike down (i.e. fall to your inside when you lean over too hard) or run off the apex of the curve because you can't lean into the curve hard enough for your speed.  Both of these things would happen before your wheel would slide out some significant amount but you wouldn't fall.

The other possibility is that you hit your rear brake too hard and locked up your rear wheel and fishtailed.

Bingo, that would be my guess too.

I think the original poster did say she was pedaling through the turns.

But it is very natural for a beginning cyclist to want to hit the brakes in a turn.  If that is what's at issue here, then my advice is to brake in a straight line before the turn if you need to bleed off some speed... although in time you'll find that cornering skill (really leaning into it on the correct line) will help much more.