Our surgery is 2 days apart - My paperwork from the dietician says no crisps until 6 months and then, as you have, only low fat ones...I can't see why you would even have Brownies in your house at this stage?
Have you tryed things like protien shakes, Scrambled egg, Phili lite, Tuna mayo, home made soup (no fat in them!) Baked sweet potato with low fat beans and cheese, there are lots and lots of things you could be having that would give you so much more goodness and nutrition than crisps and Brownies?!
I feel annoyed as it just seems like your setting yourself up to fail if you are in the mindset of "Ohh Ok lets top up with crisps and cakes..." now only 8 weeks post op?!
Each to their own, nothing to do with me really. It's up to your own personal choice. Good luck with your bloods at 3 months though with that amount of nutrition I guess you will be lacking a few of the essential vitamin and minerals to function, no wonder you felt rough this morning - no body can physically function on 3/400 cal's a day!