Mare is acting strange, is she coming into heat or breed?

We bought a mare about 6 months ago, She was a little on the heavy side but not too bad. She was place on 20 acres of good pasture and put on a little more weight but nothing alarming. We brought her up to the house where there isn't a lot of gr but always a full supply of hay (most of our horses tend to drop a few pounds when we move them here more excise and less gr) but she hasn't seemed to drop any pounds. Then about 3 days ago she started getting very nasty towards the other horse in the pasture with her and will not let you touch her at all. I went out yesterday and she was laying on her left side, at 1st I thought Colic but she got right up and walked away as soon I got close, then I thought founder but her bowel movements are good and normal color and size. Refused feed and hay and only take small sips of water at a time. She would lay down and the get up on and off all day. When I went out to feed that night she was ready to eat with no problem and munched of hay the rest of the night. Had a vet tech come and look at her who wasn't really sure what to make of it (not being able to get close to her made it a little hard to really check her out and of by the time they got there she was eating again) Was told may have just been a tummy ache, or she may be coming into heat, or maybe start of labor. She doesn't seem to have much of a bag but then again I can really get a good look. She does have a small amount of clear fluid coming from Vulva but not much. HELP PLEASE.