Have I suddenly got terrible?

It was a scorching hot day today, and I had my usual lesson with sunny, who's usually very good. Today I noticed that she was sooo impatient. I could barely slow her down. So then I was put behind someone else, and she was okay, but eager to overtake the one in front. When we were asked to trot solo at the end, she went straight into canter, and would NOT stop, untill she joined the back of the ride. I've had very limited experience of canter, but during that brief canter, I think I developed a seat for it (wasn't boncing or sliding, like before), so that was one good outcome, but for future reference, how can I slow the horse down effectively?

(btw, my stirrups felt really wierd that day, and kept slipping, which they haven't done before...)

It sounds as if she was a bit fresh! Most horses get this from time to time... sometimes if they have been off work for awhile and at other times when they simply feel the joys of spring!
Half halts can be quite effective - this is a broad term for several variations of the same effect; which one works best is really an individual preference. Personally I tend to find that applied leg pressure with a stiffening of the back accompanied with small repeated mouth pressures with the reins seems to help in slowing a hyper one down! Some are more challenging although what you say of Sunny seems that it is just an occasional hyper rather than a pro-longed spell.

Yeah, she's usually fine.

Wow, you gave the same advice as my instructor, though she said something a bit different.

Thanks for your advice.

Always nice to know you have back-up on advise!