My function always returns false the first time

This may be a bit of an obscure question, but I cannot seem to figure out what is happening here. I have a "login" function that checks a username and password against an API. For some reason, calling this function returns false the first time, but if I call the function a second time, it returns the proper true, assuming I enter the correct credentials.

Here is the function:

if testLogin(usernameTextField.text!, password: passwordTextField.text!) {
            performSegueWithIdentifier("dismissLogin", sender: self)
        } else {
            let alert = UIAlertView()
            alert.title = "Login Problem"
            alert.message = "Wrong username or password."
            alert.addButtonWithTitle("Dismiss")
            alert.show()
        }
    }

The testLogin function appears here;

func testLogin(username: String, password: String) -> Bool {

    let jsonString = "{\"username\":\"\(username)\",\"password\":\"\(password)\"}"
    let myURL = "http://myurl.com/api/login.php"

    let url:NSURL = NSURL(string: myURL)!
    let session = NSURLSession.sharedSession()

    let request = NSMutableURLRequest(URL: url)
    request.HTTPMethod = "POST"
    request.cachePolicy = NSURLRequestCachePolicy.ReloadIgnoringCacheData

    request.HTTPBody = jsonString.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: true)
    request.addValue("application/json", forHTTPHeaderField: "Content-Type")

    let task = session.dataTaskWithRequest(request) {
        (
        let data, let response, let error) in

        guard let _:NSData = data, let _:NSURLResponse = response  where error == nil else {
            print("error")
            return
        }

        do {
            let json:AnyObject? = try NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.MutableContainers)

            if let result = json as? [String: AnyObject] {
                if let status = result["Status"] as! Int? {
                    if status == 200 {
                        self.loginCheck = true
                    } else {
                        self.loginCheck = false
                    }
                }
            }
        } catch {
            print(error)
        }            
    }   
    task.resume()
    return loginCheck
}

Sorry for the length. I can't seem to fathom, though, why calling testLogin the first time returns false, but calling it the second time returns true. Almost as if the function is not calling the website fast enough and is returning false, but the second time I call the function, it has had time to check the website?