This article explains why.
"The first several weeks of pregnancy can be strange to say the least. At times you might feel completely normal; your jeans still fit, and no one can tell you're pregnant. Other times you might feel like you're moving in slow motion and about to fall asleep on your feet. And although the side effects of early pregnancy may seem freaky, rest assured that what you're feeling is almost certainly normal. How many of these classic signs and symptoms of early pregnancy do you have?
Happiness! And then sadness. And then happiness again.
Women who get PMS are more likely to have severe mood swings during pregnancy, but most women can expect some level of ricocheting emotions. Swirling and soaring hormones can make you irritable, giddy, or weepy, sometimes all at once! These mood swings are most frequent in the first trimester, and later one toward the end of your last trimester.
An aching head
Headaches are one of the biggest complaints women have during the first trimester, likely caused by hormonal changes and increased blood volume in early pregnancy. (Stress or anxiety can trigger them, too). Wait it out and they'll fade, but if you need relief now, try cold or warm compresses or a neck and shoulder massage which has been shown to relieve headache pain. And also be sure you're eating small meals regularly. Fluctuating blood sugar can trigger pregnancy headaches as well.
Breast swelling and tenderness
If you're like many women, sore breasts may have been the first clue you were pregnant! The hormones that are preparing your milk ducts for breastfeeding can also make them achey or sensitive. Investing in a larger size, very supportive bra can make you more comfy."
If you have any concerns about you or your pregnancy, please speak with your midwife.