I have found this in Babycentre:
Current research shows that an HIV-positive mother may pass the virus to her baby through breast milk. HIV causes a gradual breakdown of the immune system. The virus may eventually lead to AIDS, an often fatal condition that can cause infections of the intestinal tract, brain, lungs, and eyes, as well as extreme weight loss, diarrhea, and some types of cancer.
Can I breastfeed?
No. Medical experts have evidence that HIV can pass from an infected mother to her child through her breast milk. As a result, the World Health Organization, the American Academy of Pediatrics, the Centers for Disease Control, and the Elizabeth Glaser Pediatric Aids Foundation advise against nursing if you are HIV-positive and a safe alternative is available.
Solution
Do not pump or express your milk for your baby; instead, bottle-feed with formula.
In some cases, donated breast milk from a milk bank may be an option, especially if your baby is ill or premature. Ask your doctor about it.
Women who live in developing nations may not have access to formula, and the risk of dying of malnutrition may outweigh the risk of HIV infection. In this situation, an HIV-positive mother may have no choice but to nurse her baby.
However, you don't know yet if you are positive, and are very likely not to be. But if you have to stop breastfeeding please don't feel guilty, you are doing what's best for your child, and if BF is no longer the best option, you can rest assured that being BF for 10m is much longer than many other babies have been, and they are fine.
If you have to stop, try to relax about it as much as you can, if you are so stressed about formula it is very likely that your baby will notice and refuse to have it.
Considering she is already 10m old, I would go straight to follow-on milk. Don't be scared if things don't look well at the beginning, bottles and formula choosing depends much in the characteristics and likes of baby, so you may need to try some before you find the right ones.
Hope the appointment tonight bring you some reassurance. Best of luck.