URSSAF and early retirees

I note previous discussions on this subject. However I do not work, am an early retiree with my husband. He is 52 and I am 56. We receive small UK work pensions and are taxed in UK on these. We are both in the health system here CPAM and have lived here for over 6 years. I have been paying URSSAF since 2009, the payments remained pretty much the same for 2010 & 2011. However this year I have receied notice that the payments made 4 times a year are almost double ! Any suggestions as to why ? Yes we did have to pay a little more impot tax last year.