One thing that has similar symptoms to what your guinea is displaying is due to a Calcium deficiency. Guinea hens that are laying can develop Calcium deficiency if they are not receiving adequate calcium in their diet. The Calcium deficiency can also be due to a vitamin D deficiency which would interfere with the proper absorption of Calcium.
The symptoms will get progressively worse with loss of motor functions making it so that the guinea hen cannot even stand up or walk.
Treatment is to isolate the hen and give her calcium supplements. One method is to add liquid Calcium to the water source as a quick boost. Oyster shell should be available free choice.
Do not give the Calcium fortified water to any males as it can be harmful to their kidneys.