Sold a chihuahua puppy but is he is a x?

hi there and thanks in advance for replying.I bought a chihuahua puppy 4 weeks ago.I know something of the breed as i had my last little boy until he was 16 years old.The lady i bought him from ured me she would keep him for me until i could get there which was the following week.She is supposed to be a proper breeder,but as he is getting older i am having quite a few doubts if he is all chihuahua.There seems to be another breed in him & im thinking its jack russel.I would not part with him now as we have bonded very well.The lady did as she promised but he was the last of the litter & when i got there we were showed his parents.She did reduce him in price as i didnt need him kc registered.She also arranged the months free insurance.I would just like to know where i stand? should i ring her & ask? I also know thtat she did have a litter of chihuahua's x jacks previously.Thanks again.Or if there is any chihuahua experts out there i would love to have a chat about him,as in his appearence & other traits.