I dont understand what size i am (in the uk)?

i bought 2 pairs of jeans the other day on the internet, 1 pair was size 14, the other was size 16. When they arrived i tried the size 14 on but they wouldnt fit, i couldnt even pull them up over my bum. So then i tried the size 16 on, i pulled them up & got them over my bum, they were a bit tight over my legs but ok, but then when i was walking about in them i noticed the crotch bit was hanging low & i was able to pull them down & off without undoing the button & zip. So i dont get wot size i am, if a 14 is so tight & small but a 16 is too loose & big. Help please