He seems withdrawn from me.....what should I do?

About a month ago, one of my guy friends proposed to me and i said "yes" to him, things were going pretty well with us through internet communications as he's living abroad, but for the last 10/12 days, he seems quite withdrawn from me, he calls me up every night, but doesn't talk as much as he used to do, and doesn't even respond like before, though he doesn't misbehave or quarrell with me at all, just that he's a bit of "cold" nowdays. I've asked him if anything is wrong, but he says that nothing is wrong, but doesn't explain his "coldness" towards me. Now this is really frustrating for me because except for these phone talks, i've no other ways to communicate with him, and in this way, i don't have him as much as i want, now i need a suggestion abt what can be wrong and what to do now, thanks....