The S&P/ASX 200 (Index: ^AXJO) (ASX:XJO) has started the week off on a far more positive note after last week’s disappointing performance.

The main index has managed to climb 0.51% to 5,310 points mid-way through the session, with the strongest gains coming from the gold, materials and utilities sectors.

Despite the positive day, a number of shares have been hit hard, including:

Vita Group Limited (ASX: VTG)

Shares of Vita Group have plunged more than 11% today, most likely in response to the surprise business venture that was announced at the telecommunication retailer’s AGM last Friday. The company’s new venture, SQD Athletica, is a men’s lifestyle brand that boasts active apparel, lifestyle accessories, eyewear and skincare as its core range. Judging by today’s reaction, investors are clearly concerned that the new offering will not be complementary to its existing retail model that has proven to be so successful over recent years.

Carsales.Com Ltd (ASX: CAR)

After a pretty rough day last Friday, shares of Carsales are once again on the back foot today falling another 4% to $10.55. The company warned last week that it expects its financial and related services division to continue to under perform, which would likely result in a significant decline in first half revenue and EBITDA. Even though other segments of the business are likely to offset some of this decline, the shares were nonetheless priced for a much stronger level of overall earnings growth.

Macquarie Group Ltd (ASX: MQG)

Despite reporting better-than-expected first half results last week, shares of Macquarie Group are down more than 2.5% today after ratings agency, Standard and Poor’s (S&P), downgraded the company’s outlook from stable to negative. As highlighted here, S&P placed 25 lenders on negative watch today on the back of rising household debt and surging property prices which they believe have increased the economic risks facing the Australian financial industry.

Boral Limited (ASX: BLD)

Shares of Boral have dropped more than 2.5% today after the building materials company announced it has agreed to sell its 40% stake in the Boral / CSR Bricks joint-venture to CSR Limited (ASX: CSR) for $133.9 million. Although Boral will make a profit of $20 million to $25 million from the sale, it appears some investors believe the sale may have been a little premature with the company expected to lose around $6.5 million in earnings contribution in FY17.

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Motley Fool contributor Christopher Georges owns shares of Macquarie Group Limited. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.