Dear Sir,
Kindly elaborate your question.
 

What I have understood from your question is that the purchase have been made in Jan 2018 & you have claimed its credit in the GST return for May 2018. Whether it is allowed under GST or not.
As per the provisions of Section 16, A registered person shall not be entitled to take input tax credit in respect of any
invoice or debit note for supply of goods or services or both after the due date of furnishing
of the return under section 39 for the month of September following the end of financial year
to which such invoice or invoice relating to such debit note pertains or furnishing of the
relevant annual return, whichever is earlier.
From the above provision it can be stated that a credit is allowed earlier of the following date:

1. Due date for filing return of September of following year ( in your case it will be 20th Oct 2018)
2. Due date for filing annual return for current FY (it will be 31st dec 2018)

Earlier of these dates will be 20th Oct 2018. therefore, ITC can be claimed for FY 2017-18 till 20th Oct 2018 provided it is an eligible credit.
 
 
 

Thank very much for your answer.