Weird Behaviour

When getting excited while sitting/laying on my lap, my female cat gets on her toes, lifts her behind slightly and meows constantly. When she sits again or when I push her down, her behind twitches and she starts up again when I pet or pay attention to her. She tends to only do this when she’s on me. What’s up with this? She’s neutered and roughly 11-12 years old.

Hello!   You use the present tense as though this is something that happens regularly, all the time, but don't say whether the behaviour has been going on for 11 years, or whether it's new.  I thought of several possible explanations:   1) she has ectopic or remaining ovarian tissue somewhere, and this is sexually-related behaviour.   2) She has a tumour growing somewhere wither sinister or benign, producing sexual hormones, which is encouraging sexual behaviour    3) The cat is in pain eg spinal pain   4) This is behavioural - she has learned that this behaviour gets a certain desired response eg interaction / attention   5) she has become hyperthyroid, which can result in excessive energy with nowhere to spend it or a sudden desire for extra food.  6) Changes are happening within her brain.  This can be from primary lesions or secondary eg to liver disease (liver doesn't remove toxins, which build up, affecting the brain).     Your vet may be able to help you to differentiate between these possibilities and may have a few of their own, so it's definitely worth paying a visit.  If you change a family member or visitor to film the episode, your vet might find it extremely helpful but don't hang on for ages for this if it isn't easy - it's more important that the consultation goes ahead.    We look forward to hearing how you get on.