By the beginning of the second trimester, most women feel more energetic and their nausea begins to subside. The bad news: some women experience headaches. It's important to stay hydrated — acetaminophen usually helps. Lots of organ development is occurring in the fetus at this time and you can start seeing the external genitalia. If you have an ultrasound at this point, decide in advance whether or not you want to know the sex of the baby! On a physiology level, your circulating blood volume will rise 50% over your non-pregnant baseline, so you may become more aware of your heart beating, and you may become a bit more short of breath climbing up stairs (but do keep taking the stairs as long as you're healthy – it’s great exercise).
By the end of the second trimester, the major organ systems have developed their basic architecture, but the brain is constantly developing. You will most likely feel pretty good (no nausea or headaches); you'll just feel big. Do try to stay active, which will help minimize your "physically large feelings".