I joined this morning and I rounded up around 11 RP. Suddenly, it fell to a 7 then to a 3, then strangely back up to a 7.
Is there something I'm missing out here? Some clarity would be much appeciated.
Your user profile can be very helpful, as Frank points out. However, if you are simply trying to assess reputation, you have a button available to you across all Stack Exchange sites, including Arqade.
Pressing on this button gives you a drop down of all reputation gain/loss within a set period. Seperated into the last day, the last week, and the last month, this drop down will give you a reference of any change to your reputation within the last month. Assuming you are a regular Arqadian, this can be a quick and easy way to keep up to date with an reputation changes. You should also note that this will show you rep changes across all stack exchange sites, as you can see by my +100 to Mathematics for "being trusted".
I feel it important to note that while Frank has taken you "the long way", albeit a rather insignificant difference, he has pointed out a much more powerful analysis tool; From the user profile, you can view other elements, including your questions and answers. Each listing also gives you a good overview of the overall rating, which in turn gives you an idea on what is attracting up votes and down votes.
You recieved an upvote, which awards +10 reputation.
Then, you received several downvotes, each of which costs you -2 reputation.
The resultant net change leaves you at your current rep.