could she be getting teeth in? are there any other changes in your family's life? can you identify what her triggers are?

By allowing her to feel what she's feeling without criticism or scolding, and offering her hugs and understanding when she needs it, you ARE supporting her! It's so hard when you don't know what's going on, isn't it? Growing is difficult work & emotions are up and down! Hang in there!

I think it can be helpful to describe emotions of pre-verbal children, even learning the sign language to communicate. Possibly ask "Are you angry?" and then sign angry too. Then she will be able to express it clearer to you if she picks it up if you do it a lot. (and any other strong emotion)
My second child had a lot of this too. This age can be eye-opening for a child when they understand more but are frustrated with what they can do, even if they can't figure it out!
The 'ideal baby' is not always the most agreeable toddler. Possibly she is reacting to some outside influence, your relationship with your husband/other close family, new care enviroment etc. Even if she is worried about something that appears mundane and uninteresting to us (a new room arrangment, a new neighbor) she might be sensitive enought to react.

Its just so hard sometimes. We struggle along with food intolerances as well, so I never know if somethings upset her tum or shes just grumpy. She cant have gluten, or sulphites so is on limited foods, and is very picky with them so at times I wonder if shes just hungry as well.
She is a lovely wee girl, I just want to try get ahead of this before it escalates.

The tip about trying sign langauge is a great one, I read about it when she was much younger but didnt follow through on it. Makes sense when they arent able to talk yet.

I don't think you need to worry about getting ahead of it before it escalates. Toddlers are just learning about emotions and don't have the communication skills to express what they're feeling. My son does the same thing (18 months) and I've worked in child care and many if not most kids are also doing this. I agree with whoever suggested labeling her feelings/emotions. I have heard of a book called something like "The Emotional Life of Toddlers" or something along this line...haven't read it but heard it was good. I'd say it is really age appropriate though and you have no need to worry.