Hi Hema - I am doubtful that the price went up by $100K in 10 days. Maybe you had seen an older ad or as Helen suggested, you are comparing the base price to the final price with all options/upgrades? People do end up paying $20K - $50K higher than the base price (on average) after taking into account lot premiums, extensions, upgrades, etc.
Builders do increase prices based on demand - it can be weekly or monthly increases in price. They will go with what they can get. If they are selling quickly, they will try a little higher price and see if people are still willing to buy. But these increases are usually in the range of $5K - $10K.
At any rate, as a first time home buyer in particular, it will make the process much easier for you if you obtain the services of a knowledgeable buyer's agent - even if you are purchasing from a builder. An agent can help you analyze pricing and help if any issues come up during the transaction with the builder. I also scrutinize the builder's contract and discuss with you regarding anything that is in our regional contract but not the builder's contract. Give it a chance - try a no obligation meeting to see if you have a good repoire with an agent - I would be happy to help.