PQsendQuery/ PQgetResult Problem

I have a strange problem, or maybe it's not a strange problem but just something wrong with my understanding i have SIP router which works with postgresql using libpq, somewhere in the code it inserts a row in the database and then when the insert is finished it invokes another module which tries to read it , however sometimes the read operation doesn't find the row which was just inserted, this happens if there are large number of insert / read cycles like this are going on , the code which inserts the row uses PQsendQuery to insert and waits for PQgetResult to return null before the read module is invoked, the module which reads the row , reads it over a new connection. My question is if PQgetResult returns null is this  a good enough guarantee that a subsequent connection (even in next millisecond) should be able to read that row??

Thanks for making me wiser

/Usama