Unspayed females have been known to bond. Unfortunately, when they aren't spayed, they are more prone to have those hormones disrupt that bond. Sometimes it can even cause them to lose that bond permanently.
Keep a close eye because the aggression could escalate to outright fighting. Rabbit fights can be vicious and cause serious harm.
It might be time to consider having them both spayed. Once spayed, and healed, you can attempt to re-bond them. It will be as if they've never before met. The advantage of spaying -- aside from the potential for a more solid bond -- is that it greatly reduces the risk or cancer. Females that are left intact have a much higher incidence of getting cancer after age 5. (A healthy rabbit can live over 10 years)
If you are concerned that the one is sick and you decide to take her to the vet, be sure to bring both rabbits. Vets (rabbit savvy ones) understand that this is necessary with bonded rabbits. Never bring just one of a pair to the vet or they almost definitely will un-bond because the one will smell different (like a different rabbit) from being at the vet's.