TA cloning : clone contamination

[chukins]

I cloned a PCR product using TA cloning, I picked white colonies , 
analysed rapidly with PCR using the same primers I originally used for my 
PCR product , they were all positive.
After O/N culture in liquid media, I did  streak my white colonies, 
surprisingly I found blue colonies growing on my plates that had X-Gal.
Is there any logical explanation for this apart from the very remote 
possibility of contamination?Please help.