Inkomazi expires before expiry date all the time

this is always happening to me, now its time I complain about this, im 35 now and I have eaten inkomazi since my childhood, but Inkomazi has changed a lot, I always buy a 1L or 2L with an expiry date that is a bit far because I cannot be eating everyday, I will eat it then put in the fridge, amazingly after 2 days it starts getting sour, then the bottle expands then its turns watery and off,why? and everything in the fridge yoghurts and everything else is still okay, I thought there is something wrong with my fridge but nope, I bought this 2L last week,barcode no ***, had it for 2days, on Saturday last week it was already off and watery and the bottle has expanded and the expiry date is 13 Feb 2016( the number on the bottle is IMT6 007)..so I went to buy another brand of Amasi(didn't like it) but just wanted to see if it will do the same, its a week now and that Amasi is still okay, so there is something wrong with your product and I am not happy with this BECAUSE ITS WASTING MY MONEY, I ONLY EAT IT 2-3 TIMES AND IVE ALWAYS THROWN THE REST AWAY.what is the point of an expiry date? and it is written nowhere that once opened it must be consumed fast.i want my money back.tx