how can I tell a postposition from a case marker???

So here's my conundrum, I have a word in my conlang that has existed for a
fairly long time *indsa* and for the longest time it was always a
preposition, about equivalent to towards/near depending on whether the verb
implied motion or not.
So before recently, the scentence equivilant to "I walk towards a cat" would
be
::*vrotyomo indsa   niiaoth*
::I.walk   towards cat
but for the last little while I've been saying
::*vrotyomo niiaothindsa*
::I.walk   catwards
but it only happens with verbs implying motion, like vrotyo, for a verb that
does not imply motion it's still a preposition.
::*moktaomo indsa kufath*
::I.sit    near  dog

So my question is, since it behaves very similarly to the other case markers
like the instrumental, should I consider that for verbs of movement a new
case has evolved, or that it has become a postposition, I really can't tell.

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