Being pregnant affects your hormone levels which will make you have mood swings. Pregnancy takes a toll on the body not only physically but emotionally.
It is just your hormones changing and your feeling differently than you ever have before. When you are pregnant you are generally very tired and early on in the pregnancy you may be feeling very ill so things just tend to irritate you more easily than when you are not pregnant.
Your hormones are raging right now, usually this is worst during the first trimester. Typically by the second trimester the physical changes out weigh the emotional.
Though you are pregnant you may have the thirst to your fiance and it is quite natural and there is no wrong at all in it. But you should be aware about your physical condition and also the fact that you are pregnant. Take maximum care during your activities.