Just to clarify ... you have an existing membership active and it expires on 15th May then you bought another recently? If that's the case then your existing membership needs to run its course and the newly purchased on will only start on 15th/16th May. Buying a new membership does not prematurely terminate or supersede an existing memberhip. If you have used up your calls/texts/internet on your current membership then you might need to buy add-ons to provide these until the 15th.
Does that make sense? If not then please clarify what your online account says about your membership status, expiry date and calls/texts/internet balances.
Hope this helps.