Mare in heat?!(20............)?

Hi! I was wondering if you guys could help me. I am sorta a first time horse owner. I used to have a horse when I was 13 I am now 21. Uhmm today I was saddling up my horse Taz(mare). And I had went around her but to go let the cinch down and had seen she was kinda lifting her tail up so I kinda stood back figuring she was going to crap or fart or something. But all she did was let out this tiny bit of slim right at the place she pees out of. It looked like water but it was kinda slimmy. I was wandering if this is a cause for alarm? Or is she in heat? Or do I need a vet ASAP? More info can be given if you need it. Oh by the way she is 9 about to turn 10 and has had 8 previous foals.