If she really isnt eating really great treats and cannot follow commands she knows then its likely that she is to stressed
Its great advice to build it up slowly, lots of treats for traffic at a distance and then a step at a time
If at any point she isnt taking treats or responding to you then you are too close/too much so back off

If you have seen no other problems with her with people then you are prob right it was because she was so wound up
Take things easy tho

I cant rem where I read it but a really great tip is to watch one part of your dog, like ears, tail or mouth or something and see the changes in different situations so you can then begin to read how she is reacting to different situations and you can back off before she reacts

I've had a similar problem with my boy lunging at people/dogs while on walks through fear. I've been holding a treat right by his nose and letting him nibble at it but never quite get it all until we're past the person/dog and I've seen amazing results. He's so much better within a few weeks that I think he's forgotten he ever used to do it and although I'm persevering with the treat he doesn't really need it now. Try different tasty treats because some work where other don't (mature cheddar works for me ) and hold it right in front of his nose for her to nibble as you're walking past. I found going to the other side of the road if there's a person very helpful so as not to stress him too much, then gradually get nearer as you get more confident. Similarly with the traffic, quiet roads to start with, building up gradually with lots of tasty treats will help her make the association of traffic = nice things. Just a thought though, if she's only just been spayed she will be feeling tender and more on edge anyway which may be making the problem more obvious just at the moment.

Thanks all for the great replies Dogsey is so great for help and advice
Well today we walked down the same road again,but a shorter stretch and it was no where near as busy. And she was totally fine We passed lots of people and she completely ignored them like she usually would when playing in the park etc...So I am fairly sure that her barking at the lady last night was because she was stressed.
I have realised that it was just getting dark when we went out last night, and all the cars had their lights on, and the road was MUCH busier than it usualy is earlier in the day.
Having thought more about it she does walk down other roads with traffic,eg on the way to the woods and is usually OK, although I do know she still isn't keen on buses.I think maybe I need to get her gradually more used to slightly busier roads and roads at night.
It shows how important it is to try to understand why your dog acts the way they do though. Bless her, it really wasn't her fault she barked at the lady, it was my fault for not realising that that road would be stressful for her at that time of night.
Oh, and I went out armed with a bag of chopped up roast chicken this morning which she had no trouble enjoying as we walked along !She got rewarded for every time we walked past someone, or when lots of cars or buses went by etc.... Although I do know that when she is very stressed then any treat,no matter what, will be ignored....
So I will persevere in the things she needs a bit more help with, but thankfully feel slightly less useless today
Now as long as I can keep her calm enough not to rip her stitches ......LOL

Ben, that isa great tip about watching one part of her body...I did notice her tail was right down between her legs last night,which I know means she's scared . This morning it was up and wagging like normal !
I would love to learn more about doggy body language....can anyone recomend a good book?

Calming signals by turid rugaas (sp) is fantastic, check on YouTube for her too, there is some great vids there o what she is talking about