Turns out ...
In a cash account, buying power (BP) does not recycle after you exit a trade you opened that day. You have to wait 2 days for the stock to settle. If you apply for a margin account, your BP will return back to beginning of day after you execute a day trade. In a margin account, once you do 3 days trades in any rolling 5 days, your account is tagged as a pattern day trading account (PDT). In a cash account, there is no PDT rule.
Many thanks to Robert for the second opinion!