Congrats on breast feeding! My littlest one, of four, decided she did not want to be supplemented and refused a bottle around 2 months. Nursing is so much warmer, closer and cuddly, can you blame her! We are still breastfeeding and she is perfect :) Your baby is more efficient at getting milk from the breast than a pump, so if you are getting 1.5 oz per size when you pump, she is probably getting atleast double that by nursing. Also, your body produces up to 80% of a feeding at the time you are feeding, your baby is getting more than what is right there and ready when she latches on. By offering her a bottle, you are telling your body that it doesnt *need* to make as much milk, your breasts work on supply and demand. So, by having her nurse exclusively, she will signal to your body that you need to make more and your supply will naturally increase.
Personally I would say take the leap! If you are wanting to monitor how much she is taking in each feeding, rent/borrow an infant scale and weigh her before the feeding and then with out changing anything (diapers, all same clothes, socks etc on) plop her back on the scale and you will get an accurate number for how much she has nursed. She will self regulate on the breast better than a bottle, so you might see a lower intake when nursing. With a bottle, we tend to do the whole "come on, just one more ounce!" and wiggle the nipple around until they take some more, but with the breast, when she is done, she is done.
If she is not fussy and not hungry, having wet diapers and poops, I would say you are fine and she is getting what she needs! Also, www.kellymom.com is a great resource for all nursing questions and concerns!