Firstly I want to congratulate you on dealing with a traumatic situation so calmly. While you may not think you did at the time, I think you did extraordinarily well and your mum is lucky to have you.
FEC is a pretty tough chemo drug - I had months of this for my cancer. It's simply the pits!
I had a couple of thoughts about why this collaspe may have occurred. I agree with you about your mum's low intake of food and drink that day as I feel it was definitely a contributing factor - it's very important to keep up your fluids while on chemo even if it's just water. I know how she feels - everything simply tasted like shit (pardon my language) while on chemo, so one thing I did was have fresh lemon and lime slices in my water (in the fridge) or drink cordial. I found the water better for quenching thirst.
To try to get rid of that horrible metallic taste I used suck on barley sugar all the time - she might find this good. Another good thing is snakes (jelly lollies) or fruit tingles or even a couple of squares of chocolate. For me it seemed sweet things helped a lot.
One of my thoughts about your mum's collaspe was aside from dehydration and lack of nutrition, perhaps her white cells are very low. This can cause dizziness. Another thought is she may be anaemic - low red cells certainly makes you light headed.
Just make sure she drinks, and eats - even if she just nibbles all day long - perhaps not one big meal at the end of the day - try lots of little meals (eg: sandwich, toast with egg, milkshake) and try Sustagen - I worked for a cancer institute and we were always giving out Sustagen - it gives you great protein and helps you when your body is really low.
If it was a hot day and she had either low white or red cell (or both), was dehydrated and had hardly eaten - it's no wonder she collasped. Thank heavens she has you!
I know it's very difficult and stressful being a carer - particularly for a close relative like your parent or partner. You hate to see them so ill, but in a cruel twist of fate - we have to go thru chemo and be even more sick, before we can get better...stupid isn't it!
I would strongly recommend you or your mother get in touch with the oncologist as soon as possible and report this collaspe. They may wish to do a blood test to test her levels. Without trying to alarm you, she may need a small blood transfusion if her red cells are low. During my very long treatment I had over 8 transfusions and it's remarkable how much better I felt within 24 hrs after having this.
I do hope your mum rests up and is feeling better.
And Jess - you are a very loving daughter doing a great job!
Keep in touch
Luv Helen (aka H)