That definitely sounds like a sensitivity. It's normal to even use the same diaper for a while and then all of a sudden have the sensitivity show up... It could be that the chemicals from the diaper & inks have slowly increased over time.
Try something like 7th generation - they have no chlorine, and no coloring on them. It would be worth it to buy a small pack and switch for a week to see.
We used 7th generation in the beginning, and then a lot of other brands that we got from my baby shower until we ran out, and then started using cloth. We would get disposables when we went out or on vacation...etc. We used huggies or something for a while because the 7th gens were hard to find, more expensive...etc - and after 3-4 months, we had to switch because the inks started breaking her out all of a sudden - could have been that they chemicals built up, or it could have been a change in their manufacturing process (bad economy, trying to save costs, using lower priced materials... or just that the batch was different like any clothing dies, every batch is different...etc). Now we use 7th gen. ordered in bulk from diapers.com - no problems since - much easier and I feel more comfortable with a healthier less-chemicals diaper anyway.
On the other hand - with it being the summer months, it coud be that she is getting hotter now and sweat, possible dirt from playing outside...etc is building up and and irritating it where the diaper moves across the skin.