Hello,
The 18 month regression can be different for different LOs but often you get one or more of short naps, later naps, nap refusal, trouble settling (at BT or for naps), NWs, EWs...
I think the main cause is developmental. There can also be some SA at this time.
Some ppl find pushing nap slightly later can help (at least in terms of settling for it) and you might want to do this (as you have already tried). Others just 'muddle through' as best try can and wait for things to settle down again, although by then often a routine tweak (eg later nap) is needed anyway. If LO seems to be getting OT due to short nap then EBT can help keep that under control.
Hth. There is a support thread up above too for ppl going trough this.