OMG, are you sure you're not me??!!
My DD is also 18 weeks and is doing exactly this!! But my HV advised AGAINST weaning. Basically Formula has a lot more calories than any of the weaning foods that you could give and weaning may actually cause your LO to lose weight if she's not wanting her bottles.
My LO does have a bit of an excuse as she had thrush in her mouth which was diagnosed at 12 weeks and she'd had it pretty much since birth (of course the GPs didn't believe us because she was FF... ), so because her mouth feels so different now it's gone, her feeding has been really erratic.
I don't know about you, but I am absolutely despairing and have to walk away in tears when she doesn't take her feeds as she's dropped from the 25th to just above the 2nd centile in weight. She's happy, weeing and pooing and generally very healthy, she's also still on the right centile for height and head circumference, it's just her weight.
She's taking between 24-30oz a day at the moment and still sleeps for 12 hours a night without waking.
In pure desperation, I phoned Aptamil today on their advice line and they said not to panic. If she's taking some milk (strangely they only say they need between 6-8ozs 4-5 times a day between 4 and 5 months...) and is still weeing and pooing and is generally healthy, she will be fine. They also said that before babies start teething, their teeth begin to move around under their gums, causing pain. Because of this, the babies will only take the bare minimum to satisfy their immediate hunger as it's painful to suck. So it could be that this is happening to your LO as I think this might be the case with my DD.
As long as she is weeing and pooing and generally healthy/happy, I'm being told not to worry, so I'd say the same to you!!
HTH xxx