Memory being overwritten apparenty automatically during Get/Set

class CMyData
{
public:
	CMyData(void);
	virtual ~CMyData(void);
public:
	void Get(CMyDataItem** ppMyData, int& nSize);
	void Set(const CMyDataItem* pMyData, int nSize);
private:
	CMyDataItem* m_pMyData; // array of CMyDataItem
	int m_nTableSize;       // size of array
	CCriticalSection m_CriticalSection;
};

void CMyData::Get(CMyDataItem** ppMyData, int& nSize)
{
	CSingleLock lock(&m_CriticalSection, TRUE);
	if(NULL != m_pMyData)
	{
		if(m_nTableSize != nSize || NULL == *ppMyData)
		{
			// Table size has changed so rebuild
			delete[] *ppMyData;
			nSize = m_nTableSize;
			*ppMyData = new CMyDataItem[m_nTableSize];
		}
		CopyMemory(*ppMyData, m_pMyData, sizeof(CMyDataItem)*m_nTableSize);
	}
	else
	{
		ASSERT(0); // By context, shouldn't happen
	}
}

void CMyData::Set(const CMyDataItem* pMyData, int nSize)
{
	CSingleLock lock(&m_CriticalSection, TRUE);
	if(NULL != pMyData)
	{
		if(m_nTableSize != nSize)
		{
				// Table size has changed so rebuild
				m_nTableSize = nSize;
				delete[] m_pMyData;
				m_pMyData = new CMyDataItem[nSize];
		}
		CopyMemory(m_pMyData, pMyData, sizeof(CMyDataItem)*nSize);
	}
	else
	{
		ASSERT(0); // by context, this shouldn't happen
	}
}

	CMyDataItem* pMyData = NULL;
	m_pMyData->Get(&pMyData);
	pMyData->m_nID = 10;
	m_pMyData->Set(pMyData);
	delete[] pMyData;
	pMyData = NULL;
// repeat...

class CMyDataItem
{
public:
	CMyDataItem();
	virtual ~CMyDataItem();
public:
	int m_nSomeID;
	int m_nAnotherID;
	int m_nNumber;
	double m_dValue1;
	double m_dValue2;
	double m_dValue3;
	double m_dValue4;
	double m_dValue5;
	double m_dValue6;
	double m_dValue7;
	double m_dValue8;
	double m_dValue9;
	bool m_bBool1;
	bool m_bBool2;
	bool m_bBool3;
	unsigned int m_uiMyID;
	CString m_strString1;
	CString m_strString2;
};