Bingo Giving slots win

Last night i deposited £10 on Bingo Giving and received £7 bonus money. I decided for a change to have a couple of spins of the slots, i choose the bejeweled slots. As i had never played this before with out realising i had taken three spins at £2.50 a spin. Then i noticed my balance had gone up to £30 something. I changed my bet to 50p a spin and continued to play i got my balance up to over £50 and went to withdraw. I was able to withdraw £45 but it said if i withdrew more than £16 i would loose part or all of the bonus money that i had received in the past. I dont understand what this means, i have deposited either £40 or £50 on this site so did not think that there would be any problem with putting in a withdrawal as i have never withdrawn from this site before.

I have left the withdrawal to go ahead and see what happens. I wonder if someone can explain what this means that i will loose bonus money that i have received in the past. Will they take it off of what i have tried to withdraw?