I was rock hard that early due to BH contractions but I was having many starting from the six month of pregnancy. If you're really concerned, see your Doctor and get them to check you especially your cervix and to perform a CTG.
Not necessarily. It sounds to me like the baby has already dropped which is probably increasing pressure on your back, but it doesn't necessarily mean you will give birth early. Walking like a duck is also probably due to the baby having dropped. This can occur just before or several weeks before the baby is born. Being as this is a subsequent pregnancy for you, it's likely that your baby would drop earlier.
You say your stomach is rock hard. Do you mean all the time or are you refering to having contractions?
Ya..possibly you could be in labor already..Check with you Doc at once now..